When f.opt.MaxAge == 0, f.db is never set, however several methods later assume
it is set and attempt to access it, causing an invalid memory address error.
This change fixes the issue in a few spots (there may still be others I haven't
yet encountered.)
This fixes the Root() returned by the backend when it has returned
fs.ErrorIsFile.
Before this change it returned a root which included the file path.
Because Root() was wrong this caused the detection of the file being
moved over itself check to fail.
This adds an integration test to check it for all backends.
See: https://forum.rclone.org/t/rclone-move-chunker-dir-file-chunker-dir-deletes-all-file-chunks/43333/
Before this fix `NewObject` could return a wrapped `fs.Object(nil)`
which caused a crash. This was caused by `wrapObject` returning a
`nil` `*Object` which was cast into an `fs.Object`.
This changes the interface of `wrapObject` so it returns an
`fs.Object` instead of a `*Object` and an error which must be checked.
This forces the callers to return a `nil` object rather than an
`fs.Object(nil)`.
See: https://forum.rclone.org/t/panic-in-hasher-when-mounting-with-vfs-cache-and-not-synced-data-in-the-cache/29697/11
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
