get_device_by_label.[ch], fsck.c, util.c: New interpret_spec()

get_device_by_label.[ch], fsck.c, util.c:  New interpret_spec()
        function in get_device_by_label.c to allow the use of
        UUID= or LABEL= when creating filesystems which use external
        journal dev (e.g.  mke2fs -J device=LABEL=<journal_label>).

tune2fs.c: Use superblock s_journal_uuid to locate an external
        journal device instead of s_journal_dev when removing it.
        Allow opening journal devices to set the label and UUID
        in the ext2 superblock.

mke2fs.c, tune2fs.c: Free journal_device after use, as it is
        malloc'd in interpret spec.