* dirmngr/http.c [W32]: Include winhttp.h
(w32_get_internet_session): New.
(w32_get_proxy): New.
(get_proxy_for_url): Implement automatic proxy detection and fix error
in last patch.
(http_reinitialize): New.
* dirmngr/dirmngr.c (dirmngr_sighup_action): Call reinitialize.
* dirmngr/Makefile.am (NETLIBS) [W32]: Link with winhttp.