ipc: use shutdown(2), base atfork* callback

shutdown(2) on a socket can be preferable if there's multiple
forked processes writing to a single worker and we really want
to shut things down ASAP.

It may also be good to provide an ipc_worker_exit method which
subclasses can override if needed for graceful shutdown.  But we
won't need equivalents to atexit(3) since we can rely on DESTROY
handlers given this is Perl5.

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