Call pipeline_policy_cb when detecting pipelining.

Without this the user can't specify anything of the policy unless the
bundle is destructed (which normally doesn't happen).
Merge branch 'PATCH_5_7_pipelining__Add_CURLMOPT_PIPELINE_POLICY_FUNCTION' into PATCH_6_7_pipelining__Only_assign_NULL_to_easy_conn_directly
Merge branch 'PATCH_6_7_pipelining__Only_assign_NULL_to_easy_conn_directly' into PATCH_7_7_pipelining__Keep_used_sockets_in_pipeline_active
Merge branch 'PATCH_7_7_pipelining__Keep_used_sockets_in_pipeline_active' into PATCH_8_curl_multi_perform__Correctly_close_connection