findmnt: cleanup usage()

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: add mnt_table_uniq_fs()

Signed-off-by: Karel Zak <kzak@redhat.com>
findmnt: add -U to de-deplicate filesystems

Signed-off-by: Karel Zak <kzak@redhat.com>
fstrim: de-duplicate list of filesystems

Reported-by: Pádraig Brady <P@draigBrady.com>
Signed-off-by: Karel Zak <kzak@redhat.com>