cmd: add hashSUM file support (#5352)

Currently rclone check supports matching two file trees by sizes and hashes.
This change adds support for SUM files produced by GNU utilities like sha1sum.

Fixes #1005 

Note: checksum by default checks, hashsum by default prints sums.
New flag is named "--checkfile" but carries hash name.
Summary of introduced command forms:

```
rclone check sums.sha1 remote:path --checkfile sha1
rclone checksum sha1 sums.sha1 remote:path             
rclone hashsum sha1 remote:path --checkfile sums.sha1
rclone sha1sum remote:path --checkfile sums.sha1
rclone md5sum remote:path --checkfile sums.md5
```

docs/content/commands/rclone_check.md

@@ -24,6 +24,9 @@ both remotes and check them against each other on the fly.  This can
 be useful for remotes that don't support hashes or if you really want
 to check all the data.
 
+If you supply the `--checkfile HASH` flag with a valid hash name,
+the `source:path` must point to a text file in the SUM format.
+
 If you supply the `--one-way` flag, it will only check that files in
 the source match the files in the destination, not the other way
 around. This means that extra files in the destination that are not in
@@ -53,6 +56,7 @@ rclone check source:path dest:path [flags]
 ## Options
 
 ```
+  -C, --checkfile string        Treat source:path as a SUM file with hashes of given type
       --combined string         Make a combined report of changes to this file
       --differ string           Report all non-matching files to this file
       --download                Check by downloading rather than with hash.

docs/content/commands/rclone_checksum.md

@@ -0,0 +1,68 @@
+---
+title: "rclone checksum"
+description: "Checks the files in the source against a SUM file."
+slug: rclone_checksum
+url: /commands/rclone_checksum/
+# autogenerated - DO NOT EDIT, instead edit the source code in cmd/checksum/ and as part of making a release run "make commanddocs"
+---
+# rclone checksum
+
+Checks the files in the source against a SUM file.
+
+## Synopsis
+
+
+Checks that hashsums of source files match the SUM file.
+It compares hashes (MD5, SHA1, etc) and logs a report of files which
+don't match.  It doesn't alter the file system.
+
+If you supply the `--download` flag, it will download the data from remote
+and calculate the contents hash on the fly.  This can be useful for remotes
+that don't support hashes or if you really want to check all the data.
+
+If you supply the `--one-way` flag, it will only check that files in
+the source match the files in the destination, not the other way
+around. This means that extra files in the destination that are not in
+the source will not be detected.
+
+The `--differ`, `--missing-on-dst`, `--missing-on-src`, `--match`
+and `--error` flags write paths, one per line, to the file name (or
+stdout if it is `-`) supplied. What they write is described in the
+help below. For example `--differ` will write all paths which are
+present on both the source and destination but different.
+
+The `--combined` flag will write a file (or stdout) which contains all
+file paths with a symbol and then a space and then the path to tell
+you what happened to it. These are reminiscent of diff files.
+
+- `= path` means path was found in source and destination and was identical
+- `- path` means path was missing on the source, so only in the destination
+- `+ path` means path was missing on the destination, so only in the source
+- `* path` means path was present in source and destination but different.
+- `! path` means there was an error reading or hashing the source or dest.
+
+
+```
+rclone checksum <hash> sumfile src:path [flags]
+```
+
+## Options
+
+```
+      --combined string         Make a combined report of changes to this file
+      --differ string           Report all non-matching files to this file
+      --download                Check by hashing the contents.
+      --error string            Report all files with errors (hashing or reading) to this file
+  -h, --help                    help for checksum
+      --match string            Report all matching files to this file
+      --missing-on-dst string   Report all files missing from the destination to this file
+      --missing-on-src string   Report all files missing from the source to this file
+      --one-way                 Check one way only, source files must exist on remote
+```
+
+See the [global flags page](/flags/) for global options not listed here.
+
+## SEE ALSO
+
+* [rclone](/commands/rclone/)	 - Show help for rclone commands, flags and backends.
+

docs/content/commands/rclone_hashsum.md

@@ -48,6 +48,7 @@ rclone hashsum <hash> remote:path [flags]
 
 ```
       --base64               Output base64 encoded hashsum
+  -C, --checkfile string     Validate hashes against a given SUM file instead of printing them
       --download             Download the file and hash it locally; if this flag is not specified, the hash is requested from the remote
   -h, --help                 help for hashsum
       --output-file string   Output hashsums to a file rather than the terminal

docs/content/commands/rclone_md5sum.md

@@ -29,6 +29,7 @@ rclone md5sum remote:path [flags]
 
 ```
       --base64               Output base64 encoded hashsum
+  -C, --checkfile string     Validate hashes against a given SUM file instead of printing them
       --download             Download the file and hash it locally; if this flag is not specified, the hash is requested from the remote
   -h, --help                 help for md5sum
       --output-file string   Output hashsums to a file rather than the terminal

docs/content/commands/rclone_sha1sum.md

@@ -29,6 +29,7 @@ rclone sha1sum remote:path [flags]
 
 ```
       --base64               Output base64 encoded hashsum
+  -C, --checkfile string     Validate hashes against a given SUM file instead of printing them
       --download             Download the file and hash it locally; if this flag is not specified, the hash is requested from the remote
   -h, --help                 help for sha1sum
       --output-file string   Output hashsums to a file rather than the terminal