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
```
