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.h

@@ -23,7 +23,7 @@ struct nxt_port_mmap_tracking_s {
 };
 
 nxt_int_t
-nxt_port_mmap_get_tracking(nxt_task_t *task, nxt_port_t *port,
+nxt_port_mmap_get_tracking(nxt_task_t *task, nxt_port_mmaps_t *mmaps,
     nxt_port_mmap_tracking_t *tracking, uint32_t stream);
 
 nxt_bool_t
@@ -37,14 +37,12 @@ nxt_bool_t
 nxt_port_mmap_tracking_read(nxt_task_t *task, nxt_port_recv_msg_t *msg);
 
 /*
- * Allocates nxt_but_t structure from port's mem_pool, assigns this buf 'mem'
- * pointers to first available shared mem bucket(s). 'size' used as a hint to
- * acquire several successive buckets if possible.
- *
- * This function assumes that current thread operates the 'port' exclusively.
+ * Allocates nxt_but_t structure from task's thread engine mem_pool, assigns
+ * this buf 'mem' pointers to first available shared mem bucket(s). 'size'
+ * used as a hint to acquire several successive buckets if possible.
  */
 nxt_buf_t *
-nxt_port_mmap_get_buf(nxt_task_t *task, nxt_port_t *port, size_t size);
+nxt_port_mmap_get_buf(nxt_task_t *task, nxt_port_mmaps_t *mmaps, size_t size);
 
 nxt_int_t nxt_port_mmap_increase_buf(nxt_task_t *task, nxt_buf_t *b,
     size_t size, size_t min_size);