http: enable keepalive on TCP sockets

http: enable keepalive on TCP sockets

This is a follow up to commit e47a8583 (enable SO_KEEPALIVE for
connected TCP sockets, 2011-12-06).

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.

I noticed this problem with some non-interactive cronjobs getting
hung when talking to HTTP servers.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>