Compile multiple network backends into the binary and choose which one to use at runtime.

This makes it possible to share more code between the backends and also work around
bugs of certain backends on some (versions of) operating systems.

src/network/network.c

@@ -47,7 +47,13 @@ int net_initialize()
 		}
 #endif /* WINSOCK */
 
-		net_backend_initialize();
+		if (!net_backend_init())
+		{
+#ifdef WINSOCK
+			WSACleanup();
+#endif
+			return -1;
+		}
 		net_stats_initialize();
 
 #ifdef SSL_SUPPORT