Introducing the shared application port.

This is the port shared between all application processes which use it to pass
requests for processing.  Using it significantly simplifies the request
processing code in the router.  The drawback is 2 more file descriptors per each
configured application and more complex libunit message wait/read code.

src/nxt_unit.h

@@ -202,8 +202,21 @@ nxt_unit_ctx_t *nxt_unit_init(nxt_unit_init_t *);
  */
 int nxt_unit_run(nxt_unit_ctx_t *);
 
+int nxt_unit_run_ctx(nxt_unit_ctx_t *ctx);
+
+int nxt_unit_run_shared(nxt_unit_ctx_t *ctx);
+
+/*
+ * Receive and process one message, invoke configured callbacks.
+ *
+ * If application implements it's own event loop, each datagram received
+ * from port socket should be initially processed by unit.  This function
+ * may invoke other application-defined callback for message processing.
+ */
 int nxt_unit_run_once(nxt_unit_ctx_t *ctx);
 
+int nxt_unit_process_port_msg(nxt_unit_ctx_t *ctx, nxt_unit_port_t *port);
+
 /* Destroy application library object. */
 void nxt_unit_done(nxt_unit_ctx_t *);