http: allow gssapi credential delegation

If either the config option http.gssDelegate or the environment variable
GIT_CURL_GSSDELEGATE are set, enable gssapi credential delegation in libCURL
with the CURLGSSAPI_DELEGATION_FLAG option. This allows git to delegate
credentials to the remote web server to be used to access protected resources
which may be needed to access files or other resources.

Similarly, if either the config option http.gssDelegatePolicy or the environment
variable GIT_CURL_GSSDELEGATEPOLICY are set, enable gssapi credential delegation
in libCURL with the CURLGSSAPI_DELEGATION_POLICY_FLAG option.  This allows git
to delegate credentials to the remote web server if policy allows to be used to
access protected resources which may be needed to access files or other
resources.