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.

go/nxt_cgo_lib.h

@@ -35,6 +35,8 @@ int nxt_cgo_request_close(uintptr_t req);
 
 void nxt_cgo_request_done(uintptr_t req, int res);
 
+void nxt_cgo_unit_run_shared(uintptr_t ctx);
+
 void nxt_cgo_warn(uintptr_t msg, uint32_t msg_len);
 
 #endif /* _NXT_CGO_LIB_H_INCLUDED_ */