Add support for excluding cache directories

[perillo]

See http://www.brynosaurus.com/cachedir/.

Some tools that support the specification are:

• Restic, with --exclude-caches
• tar, with --exclude-caches, --exclude-caches-all and --exclude-caches-under
• Kopia

[idallen]

Yes, rsync would benefit from obeying the CACHEDIR.TAG that other programs obey, such as GNU tar. Saying that you can simulate the same thing by adding rsync-specific filter files everywhere isn't helpful since that (1) doubles the number of files one has to add everywhere, (2) opens the security risk of --trust-sender, and (3) only works for rsync.

This thread got duplicated at [#499]feature request: support Cache Directory Tagging Specification