sm-notify: factor socket creation out of notify()

sm-notify: factor socket creation out of notify()

The top half of the notify() function creates the main socket that
sm-notify uses to do its job.  To make adding IPv6 support simpler,
refactor that piece into a separate function.

The logic is modified slightly so that exit(3) is invoked only in
main().  This is not required, but it makes the code slightly easier
to understand and maintain.

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