I would like to be able to name a few remotes that must retain *all* annexed
files.  `git-annex fsck` should warn me if any files are missing from those
remotes, even if `annex.numcopies` has been satisfied by other remotes.

I imagine this could also be useful for bup remotes, but I haven't actually
looked at those yet.

Based on existing output, this is what a warning message could look like:

	fsck FILE
		3 of 3 trustworthy copies of FILE exist.
		FILE is, however, still missing from these required remotes:
			UUID -- Backup Drive 1
			UUID -- Backup Drive 2
		Back it up with git-annex copy.
	Warning

What do you think?
