Set proxy override with http_init()

Set proxy override with http_init()

In transport.c, proxy setting (the one from the remote conf) was set through
curl_easy_setopt() call, while http.c already does the same with the
http.proxy setting. We now just use this infrastructure instead, and make
http_init() now take the struct remote as argument so that it can take the
http_proxy setting from there, and any other property that would be added
later.

At the same time, we make get_http_walker() take a struct remote argument
too, and pass it to http_init(), which makes remote defined proxy be used
for more than get_refs_via_curl().

We leave out http-fetch and http-push, which don't use remotes for the
moment, purposefully.

Signed-off-by: Mike Hommey <mh@glandium.org>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>