Removing Unix control socket on start failure.

The bug had appeared in 5cc5002a788e when process type has been
converted to bitmask. This commit reverts the type back to a number.

This commit is related to #131 issue on GitHub.

src/nxt_port.c

@@ -306,8 +306,6 @@ nxt_port_process_ready_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
 
     rt = task->thread->runtime;
 
-    nxt_assert(nxt_runtime_is_main(rt));
-
     process = nxt_runtime_process_find(rt, msg->port_msg.pid);
     if (nxt_slow_path(process == NULL)) {
         return;