connect: let callers know if connection is a socket

connect: let callers know if connection is a socket

They might care because they want to do a half-duplex close.
With pipes, that means simply closing the output descriptor;
with a socket, you must actually call shutdown.

Instead of exposing the magic no_fork child_process struct,
let's encapsulate the test in a function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>