Adds Win32 support to the GIO network status monitor api. Ported from Pidgin.

The code is set up as either a loadable GIOModule or be built in-tree (with the -DNOT_A_MODULE flag) and should work for GIO versions between 2.32 and 2.46, (and any future versions of GIO as long as no new required API is added to GNetworkMonitor)

Compiling with mingw requires the -lws2_32 compiler flag