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

etc/rc.d/fsck

@@ -57,7 +57,7 @@ fsck_start()
 			echo "Reboot failed; help!"
 			stop_boot
 			;;
-		8)
+		8|16)
 			if checkyesno fsck_y_enable; then
 				echo "File system preen failed, trying fsck -y ${fsck_y_flags}"
 				fsck -y ${fsck_y_flags}