enable SO_KEEPALIVE for connected TCP sockets

enable SO_KEEPALIVE for connected TCP sockets

Sockets may never receive notification of some link errors,
causing "git fetch" or similar processes to hang forever.
Enabling keepalive messages allows hung processes to error out
after a few minutes/hours depending on the keepalive settings of
the system.

This is a problem noticed when running non-interactive
cronjobs to mirror repositories using "git fetch".

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>