Fix mingw32 build warnings

Work around buffer and ioctlsocket argument type signedness problems
Suppress a prototype which is unused on mingw32
Expand a macro to avoid warnings from some GCC versions

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

slirp/socket.c

@@ -474,7 +474,12 @@ sorecvfrom(struct socket *so)
 	  udp_detach(so);
 	} else {                            	/* A "normal" UDP packet */
 	  struct mbuf *m;
-	  int len, n;
+          int len;
+#ifdef _WIN32
+          unsigned long n;
+#else
+          int n;
+#endif
 
 	  if (!(m = m_get())) return;
 	  m->m_data += IF_MAXLINKHDR;