sm-notify: Use getaddrinfo(3) to create bind address in smn_create_socket()

This patch updates the "bind to a user-specified port" arm of
smn_create_socket() so it can deal with IPv6 bind addresses.

A single getaddrinfo(3) call can convert a user-specified bind address
or hostname to a socket address, optionally plant a provided port
number, or whip up an appropriate wildcard address for use as the main
socket's bind address.

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