http: control GSSAPI credential delegation

http: control GSSAPI credential delegation

Delegation of credentials is disabled by default in libcurl since
version 7.21.7 due to security vulnerability CVE-2011-2192. Which
makes troubles with GSS/kerberos authentication when delegation
of credentials is required. This can be changed with option
CURLOPT_GSSAPI_DELEGATION in libcurl with set expected parameter
since libcurl version 7.22.0.

This patch provides new configuration variable http.delegation
which corresponds to curl parameter "--delegation" (see man 1 curl).

The following values are supported:

* none (default).
* policy
* always

Signed-off-by: Petr Stodulka <pstodulk@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>