Minor background cleanups:

1. Set the FS_NEEDSFSCK flag when unexpected problems are encountered.
2. Clear the FS_NEEDSFSCK flag after a successful foreground cleanup.
3. Refuse to run in background when the FS_NEEDSFSCK flag is set.
4. Avoid taking and removing a snapshot when the filesystem is already clean.
5. Properly implement the force cleaning (-f) flag when in preen mode.

Note that you need to have revision 1.21 (date: 2001/04/14 05:26:28) of
fs.h installed in <ufs/ffs/fs.h> defining FS_NEEDSFSCK for this to compile.