Introducing named port message handlers to avoid misprints.

src/nxt_main.h

@@ -19,6 +19,7 @@ typedef struct nxt_port_s            nxt_port_t;
 typedef struct nxt_task_s            nxt_task_t;
 typedef struct nxt_port_recv_msg_s   nxt_port_recv_msg_t;
 typedef void (*nxt_port_handler_t)(nxt_task_t *task, nxt_port_recv_msg_t *msg);
+typedef struct nxt_port_handlers_s   nxt_port_handlers_t;
 typedef struct nxt_sig_event_s       nxt_sig_event_t;
 typedef struct nxt_runtime_s         nxt_runtime_t;