Add an FFS specific mount option to allow a filesystem checker

Add an FFS specific mount option to allow a filesystem checker
(typically fsck_ffs) to register that it wishes to use FFS specific
sysctl's to update the filesystem. This ensures that two checkers
cannot run on a given filesystem at the same time and that no other
process accidentally or maliciously uses the filesystem updating
sysctls inappropriately. This functionality is needed by the
journaling soft-updates recovery code.