Factor out mg_alloc_conn()

src/net.h

@@ -77,3 +77,6 @@ char *mg_ntoa(const struct mg_addr *addr, char *buf, size_t len);
 
 struct mg_connection *mg_mkpipe(struct mg_mgr *, mg_event_handler_t, void *);
 void mg_mgr_wakeup(struct mg_connection *pipe, const void *buf, size_t len);
+
+// These functions are used to integrate with custom network stacks
+struct mg_connection *mg_alloc_conn(struct mg_mgr *, bool is_client, void *fd);