dirmngr: Fix setup of libdns for W32.
* configure.ac (DNSLIB) {W32]: Add -liphlpapi.
* dirmngr/dns-stuff.c [W32]: Include iphlpapi.h and define
WIN32_LEAN_AND_MEAN.
(libdns_init) [W32]: Use GetNetworkParams to get the nameserver.
* dirmngr/t-dns-stuff.c (init_sockets): New.
(main): Call it.- Signed-off-by: Werner Koch <wk@gnupg.org>