patch, 3 partshttp: proxy support improvements

[cc'ing jeff king and sam vilain as per git-blame they've work on http auth and proxy code..]

Hi, my initial motivation for this patch was to add NTLM proxy authentication so I could 'git clone' from inside my employers network, but apart from doing that, I also added two more features, so, so I'm sending three patches which implement the following features:

- Support NTLM proxy authentication (as well as other authentication
methods) by setting CURLOPT_PROXYAUTH[1] to CURLAUTH_ANY.

- Look up environment vars http_proxy and HTTP_PROXY in case git
http.proxy config option is not set. This supports system wide proxy
support in terminals.

- Support proxy urls with username but without a password, in which
case we interactively ask for the password (as it's already done in
http auth code). This makes possible to not have the password written
down in git config files or in env vars.

Thanks!

[1] http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTPROXYAUTH

Nelson Benitez Leon (3):
  http: authenticate on NTLM proxies and others suppported by CuRL
  http: try standard proxy env vars when http.proxy config option is
    not set
  http: when proxy url has username but no password, ask for password

 http.c |   50 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 files changed, 48 insertions(+), 2 deletions(-)