DragonFlyBSD

The periodical "hammer cleanup" run doesn't handle all active PFS. It scans
through the mount points of fstype "hammer" or "null" whose device starts with
/pfs.
I have two hammer filesystems, and keep the pfs symlinks in their respective
root directories, so "/pfs" didn't match "/backup/pfs".
Attached patch changes the test to look for the occurrence of "/@@0x" in the
device path, which should be rare enough to only match hammer filesystems.