mount.nfs: Fix issue with -o user,exec

It would seem to make sense for mount.nfs to impose the
  "-o user" => "-o noexec,nodev,nosuid"
rule.  However if you give "user,exec" to /sbin/mount,
it will pass down
    nodev,nosuid,user
with the 'exec' flag :-(

So we have to leave that handling of that particular rule to
/sbin/mount.