Bandaid for mount_nfs segfaulting with the more obscure mount options

Bandaid for mount_nfs segfaulting with the more obscure mount options
in /etc/fstab.  This isn't a real fix though and I'm still not sure
why it started failing.  mount(8) breaks up the nfs args into seperate
repeated '-o option=value' arguments.  But, the altflags variable that
we use to track things is incrementally built up each time we see the
next option and shows us the cumulative set of flags, not just the
flag that we are currently looking at.  As a result, the strstr hack
for looking up flags in a giant -o opt=val,opt=val, etc string was failing
and causing a segfault.  I do not know what changed recently that caused
this to suddenly break, but the code has been rather bogus for some time.