Process stop notification from master to all other processes.

New port message type introduced NXT_PORT_MSG_REMOVE_PID. Default handler
removes process description from nxt_runtime_t with all ports, incoming and
outgoing mmaps etc.

src/nxt_worker_process.c

@@ -28,6 +28,7 @@ nxt_port_handler_t  nxt_worker_process_port_handlers[] = {
     nxt_port_change_log_file_handler,
     nxt_port_mmap_handler,
     nxt_port_data_handler,
+    nxt_port_remove_pid_handler,
 };
 
 
@@ -37,6 +38,7 @@ nxt_port_handler_t  nxt_app_process_port_handlers[] = {
     nxt_port_change_log_file_handler,
     nxt_port_mmap_handler,
     nxt_port_app_data_handler,
+    nxt_port_remove_pid_handler,
 };
 
 
@@ -46,6 +48,7 @@ nxt_port_handler_t  nxt_router_process_port_handlers[] = {
     nxt_port_change_log_file_handler,
     nxt_port_mmap_handler,
     nxt_router_conf_data_handler,
+    nxt_port_remove_pid_handler,
 };