rsync-backup.{in,8}: Introduce `hook' concept.

I want to make the configuration file so that it can be read by
scripts which don't actually want to run a backup.  Therefore, code
which currently runs unconditionally to (e.g.) copy metadata onto the
backup volume must be wrapped up in a function.

To make this a bit more disciplined, introduce a general idea of hooks
which are run with arguments.  The existing `backup_precommit_hook'
and `backup_commit_hook' functions are subsumed by this, still
provided for compatibility.  New hooks are added at the start of the
whole backup, at the beginning of each filesystem dump, and at the end
of the whole operation.