Changing router to application shared memory exchange protocol.

The application process needs to request the shared memory segment from the
router instead of the latter pushing the segment before sending a request to
the application.  This is required to simplify the communication between the
router and the application and to prepare the router for using the application
shared port and then the queue.

src/nxt_port_memory_int.h

@@ -63,6 +63,7 @@ struct nxt_port_mmap_header_s {
 struct nxt_port_mmap_handler_s {
     nxt_port_mmap_header_t  *hdr;
     nxt_atomic_t            use_count;
+    nxt_fd_t                fd;
 };
 
 /*