Add quitclean command to fsdb(8) to request a filesystem not be marked dirty.

A new command, quitclean, is added to fsdb(8) to request that the
filesystem not be marked as needing a full fsck(8). This is useful
when creating deliberately bad filesystem images to be used to check
that fsck is properly able to clean them up.

MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation