nfs mounts don't work through fstab

[Dominic Fandrey]

My nfs mounts haven't been working by their fstab entries, ever since I 
switched to RELENG_7. Today I took the time tracking the issue down by 
adding lots of printf to mount.c. I tracked the problem down to the function
fstabscan() in the file "/usr/src/lib/libc/gen/fstab.c".

The result of my testing was that it is now obligatory to set 'sw', 'ro' or 
'rw' in the options list.

This is a change, that in my opinion should either be undone or documented 
in "/usr/src/UPDATING" and in the fstab(5) manual page.