statd: Support TI-RPC statd listener

If TI-RPC is available, use it to create statd's svc listener.  If
not, use the old function, rpc_init(), to create statd's listener.

IPv6 can be supported if TI-RPC is available.  In this case,
/etc/netconfig is searched to determine which transports to advertise.

Add the new listener creation API in libnfs.a since other components
of nfs-utils (such as rpc.mountd) will eventually want to share it.

A little re-arrangement of when the statd listener is created is done
to make unregistration of the statd service more reliable.  As it is
now, the statd service is never unregistered when it exits.  After it
is gone, other programs usually hang when trying to access statd or
see if it's running, since the registration is still there but statd
itself does not respond.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>