Modify rc.d/fsck to handle new status from fsck/fsck_ffs

r328013 introduced a new error code from fsck_ffs that indicates that
it could not completely fix the file system; this happens when it
prints the message PLEASE RERUN FSCK. However, this status can happen
when fsck is run in "preen" mode and the rc.d/fsck script does not
handle that error code. Modify rc.d/fsck so that if "fsck -p"
("preen") returns the new status code (16) it will run "fsck -y", as
it currently does for a status code of 8 (the "standard error exit").

Reported by: markj
Reviewed by: mckusick, markj, ian, rgrimes
MFC after: 3 days
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D14679