Port RPC interface introduced.

Usage:
1. Register handlers in incoming port with nxt_port_rpc_register_handler().
2. Use return value as a stream identifier for next nxt_port_socket_write().

src/nxt_worker_process.c

@@ -29,6 +29,9 @@ nxt_port_handler_t  nxt_controller_process_port_handlers[] = {
     nxt_port_mmap_handler,
     nxt_port_controller_data_handler,
     nxt_port_remove_pid_handler,
+    NULL,
+    nxt_port_rpc_handler,
+    nxt_port_rpc_handler,
 };
 
 
@@ -39,6 +42,9 @@ nxt_port_handler_t  nxt_worker_process_port_handlers[] = {
     nxt_port_mmap_handler,
     nxt_port_data_handler,
     nxt_port_remove_pid_handler,
+    NULL,
+    nxt_port_rpc_handler,
+    nxt_port_rpc_handler,
 };
 
 
@@ -49,6 +55,9 @@ nxt_port_handler_t  nxt_app_process_port_handlers[] = {
     nxt_port_mmap_handler,
     nxt_port_app_data_handler,
     nxt_port_remove_pid_handler,
+    NULL,
+    nxt_port_rpc_handler,
+    nxt_port_rpc_handler,
 };
 
 
@@ -59,6 +68,9 @@ nxt_port_handler_t  nxt_router_process_port_handlers[] = {
     nxt_port_mmap_handler,
     nxt_router_conf_data_handler,
     nxt_port_remove_pid_handler,
+    NULL,
+    nxt_port_rpc_handler,
+    nxt_port_rpc_handler,
 };