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_unit.h

@@ -137,6 +137,9 @@ struct nxt_unit_callbacks_s {
     /* Gracefully quit the application. Optional. */
     void     (*quit)(nxt_unit_ctx_t *);
 
+    /* Shared memory release acknowledgement. */
+    void     (*shm_ack_handler)(nxt_unit_ctx_t *);
+
     /* Send data and control to process pid using port id. Optional. */
     ssize_t  (*port_send)(nxt_unit_ctx_t *, nxt_unit_port_id_t *port_id,
                  const void *buf, size_t buf_size,
@@ -323,6 +326,9 @@ uint32_t nxt_unit_buf_min(void);
 int nxt_unit_response_write(nxt_unit_request_info_t *req, const void *start,
     size_t size);
 
+ssize_t nxt_unit_response_write_nb(nxt_unit_request_info_t *req,
+    const void *start, size_t size, size_t min_size);
+
 int nxt_unit_response_write_cb(nxt_unit_request_info_t *req,
     nxt_unit_read_info_t *read_info);