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.h

@@ -44,6 +44,9 @@ struct nxt_port_handlers_s {
 
     /* Various data. */
     nxt_port_handler_t  data;
+
+    nxt_port_handler_t  oosm;
+    nxt_port_handler_t  shm_ack;
 };
 
 
@@ -82,6 +85,9 @@ typedef enum {
 
     _NXT_PORT_MSG_DATA          = nxt_port_handler_idx(data),
 
+    _NXT_PORT_MSG_OOSM          = nxt_port_handler_idx(oosm),
+    _NXT_PORT_MSG_SHM_ACK       = nxt_port_handler_idx(shm_ack),
+
     NXT_PORT_MSG_MAX            = sizeof(nxt_port_handlers_t)
                                   / sizeof(nxt_port_handler_t),
 
@@ -114,6 +120,9 @@ typedef enum {
 
     NXT_PORT_MSG_DATA           = _NXT_PORT_MSG_DATA,
     NXT_PORT_MSG_DATA_LAST      = _NXT_PORT_MSG_DATA | NXT_PORT_MSG_LAST,
+
+    NXT_PORT_MSG_OOSM           = _NXT_PORT_MSG_OOSM | NXT_PORT_MSG_LAST,
+    NXT_PORT_MSG_SHM_ACK        = _NXT_PORT_MSG_SHM_ACK | NXT_PORT_MSG_LAST,
 } nxt_port_msg_type_t;