Back out previous delta to fix fsck on filesystems without an fstab entry,

Back out previous delta to fix fsck on filesystems without an fstab entry,
where we want to take the disklabel filesystem type of "4.2BSD" and use
fsck_4.2bsd on those filesystems.

Add a comment about why the code is there, now that we know:

         * XXX This is a kludge to make automatic filesystem type guessing
         * from the disklabel work for "4.2BSD" filesystems.  It does a
         * very limited subset of transliteration to a normalised form of
         * filesystem name, and we do not seem to enforce a filesystem
         * name character set.