Mountd listens on 2 different versions for NFSv2 (MOUNTv1 and MOUNTv2)

Mountd listens on 2 different versions for NFSv2 (MOUNTv1 and MOUNTv2)
and one for NFSv3 (MOUNTv3)

When --no-nfs-version requests an NFS version to be disabled, the
code actually disabled the MOUNT version. This works is several cases,
but requires --no-nfs-version 1 to completely disable NFSv2, which
is wrong.

So if we do disable 1, 2, and 3. mountd complain and won't run, it
is not possible to run just v4 - i.e. not listening for MOUNT requests
at all (as v4 doesn't need them).

So change the handling of "--no-nfs-version 2" it disable MOUNTv1 as
well as
MOUNTv2, and allow mountd to continue running as long as one of
NFSv2 NFSv3 NFSv4 is enabled.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>