Introducing src_pid for mmap header for accurate buf completion.

This allows to use shared memory to communicate with main process.

This patch changes shared memory segment format and breaks compatibility
with older modules.

src/nxt_port_memory_int.h

@@ -48,7 +48,8 @@ typedef nxt_atomic_uint_t  nxt_free_map_t;
 /* Mapped at the start of shared memory segment. */
 struct nxt_port_mmap_header_s {
     uint32_t        id;
-    nxt_pid_t       pid; /* For sanity check. */
+    nxt_pid_t       src_pid; /* For sanity check. */
+    nxt_pid_t       dst_pid; /* For sanity check. */
     nxt_port_id_t   sent_over;
     nxt_free_map_t  free_map[MAX_FREE_IDX];
 };