- don't set the clean flag on unmount of an unclean filesystem that was
(forcibly) mounted rw.
- set the clean flag on rw -> ro update of a mounted initially-clean
filesystem.
- fixed some style bugs (mostly long lines).
This uses the fs_flags field and FS_UNCLEAN state bit which were
introduced in the softdep changes. NetBSD uses extra state bits in
fs_clean.
Reviewed by: luoqui