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
```
In f544234 we removed the global flags from each command as it was
making each page very big and causing 1000s of lines of duplication in
the man page.
This change adds a new flags page with all the global flags on and
links each command page to it.
Fixes#3273
* put the most up to date docs into the code
* generate command docs using rclone gendocs
* put command docs into own directory
* remake them into MANUAL.md
