nfs-utils: add IPv6 support to nfsd

Add support for handing off IPv6 sockets to the kernel for nfsd. One of
the main goals here is to not change the behavior of options and not to
add any new ones, so this patch attempts to do that.

We also don't want to break anything in the event that someone has an
rpc.nfsd program built with IPv6 capability, but the knfsd doesn't
support IPv6. Ditto for the cases where IPv6 is either not compiled in
or is compiled in and blacklisted.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>