http: pass http.cookiefile using CURLOPT_COOKIEFILE

http: pass http.cookiefile using CURLOPT_COOKIEFILE

If the config option http.cookiefile is set, pass this file to libCURL using
the CURLOPT_COOKIEFILE option. This is similar to calling curl with the -b
option.  This allows git http authorization with authentication mechanisms
that use cookies, such as SAML Enhanced Client or Proxy (ECP) used by
Shibboleth.

To use SAML/ECP, the user needs to request a session cookie with their own ECP
code. See for example:

<https://wiki.shibboleth.net/confluence/display/SHIB2/ECP>

Once the cookie file has been created, it can be passed to git with, e.g.

git config --global http.cookiefile "/home/dbrown/.curlcookies"

libCURL will then pass the appropriate session cookies to the git http server.

Signed-off-by: Duncan Brown <duncan.brown@ligo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>