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_main.h

@@ -126,6 +126,7 @@ nxt_thread_extern_data(nxt_thread_t, nxt_thread_context);
 
 #include <nxt_port.h>
 #include <nxt_port_memory.h>
+#include <nxt_port_rpc.h>
 #if (NXT_THREADS)
 #include <nxt_thread_pool.h>
 #endif