http: add http.savecookies option to write out HTTP cookies

http: add http.savecookies option to write out HTTP cookies

HTTP servers may send Set-Cookie headers in a response and expect them
to be set on subsequent requests. By default, libcurl behavior is to
store such cookies in memory and reuse them across requests within a
single session. However, it may also make sense, depending on the
server and the cookies, to store them across sessions. Provide users
an option to enable this behavior, writing cookies out to the same
file specified in http.cookiefile.

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>