small mount_mfs nit

[Ted Unangst]

There's no reason mount_mfs can't handle force mounts.  Without this, -f 
just prints a silly warning.

Index: newfs.c
===================================================================
RCS file: /home/tedu/cvs/src/sbin/newfs/newfs.c,v
retrieving revision 1.86
diff -u -r1.86 newfs.c
--- newfs.c     2 Jul 2010 22:02:48 -0000       1.86
+++ newfs.c     22 Jul 2010 23:40:32 -0000
@@ -75,6 +75,7 @@
        MOPT_STDOPTS,
        MOPT_ASYNC,
        MOPT_UPDATE,
+       MOPT_FORCE,
        { NULL },
 };