git-update-dirs: Split "ga move --unused" into -U/--ga-moveunused

Bad idea to let -G/--ga-dropget move unused contents to $archive_disk as
it doesn't check that there's enough copies of old unused data. Instead
make this optional by using -U/--ga-moveunused.

be5fa650-01f9-11e4-9b59-c80aa9e67bbd
git-update-dirs: Add -u/--ga-dropunused option

This is a safer version of the -U/--ga-moveunused option, that's why
it's lower case -u instead of -U. "git-annex dropunused all" checks that
there's enough copies as defined by "git-annex numcopies" before
deleting old contents, whereas "git-annex move --unused" does not.

9651b9a4-01fa-11e4-9ed4-c80aa9e67bbd