http.c: make finish_active_slot() and handle_curl_result() static

http.c: make finish_active_slot() and handle_curl_result() static

They used to be used directly by remote-curl.c for the smart-http
protocol. But they got wrapped by run_one_slot() in beed336 (http:
never use curl_easy_perform, 2014-02-18).  Any future users are
expected to follow that route.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>