remote-http(s): support SOCKS proxies

remote-http(s): support SOCKS proxies

With this patch we properly support SOCKS proxies, configured e.g. like
this:

	git config http.proxy socks5://192.168.67.1:32767

Without this patch, Git mistakenly tries to use SOCKS proxies as if they
were HTTP proxies, resulting in a error message like:

	fatal: unable to access 'http://.../': Proxy CONNECT aborted

This patch was required to work behind a faulty AP and scraped from
http://stackoverflow.com/questions/15227130/#15228479 and guarded with
an appropriate cURL version check by Johannes Schindelin.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>