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.