Events: made a failure to create a notification channel non-fatal.

This may happen if eventfd() returns ENOSYS, notably seen on CentOS 5.4.
Such a failure will now just disable the notification mechanism and let
the callers cope with it, instead of failing to start worker processes.
If thread pools are not configured, this can safely be ignored.

src/event/modules/ngx_epoll_module.c

@@ -329,7 +329,7 @@ ngx_epoll_init(ngx_cycle_t *cycle, ngx_msec_t timer)
 
 #if (NGX_HAVE_EVENTFD)
         if (ngx_epoll_notify_init(cycle->log) != NGX_OK) {
-            return NGX_ERROR;
+            ngx_epoll_module_ctx.actions.notify = NULL;
         }
 #endif