libnsm.a: Add RPC construction helper functions

To manage concurrency, both statd and sm-notify construct raw RPC
requests in socket buffers, and use a minimal request scheduler
to send these requests and manage replies.  Both statd and sm-notify
open code the RPC request construction.

Introduce helper functions that can construct and send raw
NSMPROC_NOTIFY, NLM downcalls, and portmapper calls over a datagram
socket, and receive and parse their replies.  Support for IPv6 and
RPCB_GETADDR is featured.  This code (and the IPv6 support it
introduces) can now be shared by statd and sm-notify, eliminating
code and bug duplication.

This implementation is based on what's in utils/statd/rmtcall.c now,
but is wrapped up in a nice API and includes extra error checking.

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