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/connection.c

@@ -249,6 +249,11 @@ void* net_con_get_ptr(struct net_connection* con)
 	return con->ptr;
 }
 
+void net_con_destroy(struct net_connection* con)
+{
+	hub_free(con);
+}
+
 void net_con_callback(struct net_connection* con, int events)
 {
 	if (con->flags & NET_CLEANUP)