Fix compiler warnings in w32.c

Fix compiler warnings in w32.c

* src/w32.c (sys_socket): In case of error, use -1 as return
value, not INVALID_SOCKET, which causes compiler warnings.
(maybe_load_unicows_dll): Cast the return value of GetProcAddress
to the appropriate function signature, to avoid compiler errors.
Reported by Andy Moreton <andrewjmoreton@gmail.com>.  (Bug#21953)