Introducing port messages to notify about out of shared memory.

- OOSM (out of shared memory).  Sent by application process to router
  when application reaches the limit of allocated shared memory and
  needs more.
- SHM_ACK.  Sent by router to application when the application's shared
  memory is released and the OOSM flag is enabled for the segment.

This implements blocking mode (the library waits for SHM_ACK in case of
out of shared memory condition and retries allocating the required memory
amount) and non-blocking mode (the library notifies the application that
it's out of shared memory and returns control to the application module
that sets up the output queue and puts SHM_ACK in the main message loop).

src/nxt_port_memory_int.h

@@ -51,6 +51,7 @@ struct nxt_port_mmap_header_s {
     nxt_pid_t       src_pid; /* For sanity check. */
     nxt_pid_t       dst_pid; /* For sanity check. */
     nxt_port_id_t   sent_over;
+    nxt_atomic_t    oosm;
     nxt_free_map_t  free_map[MAX_FREE_IDX];
     nxt_free_map_t  free_map_padding;
     nxt_free_map_t  free_tracking_map[MAX_FREE_IDX];