util: add a function to check output data in a file descriptor

We can't dump netlink socket, inotify, fanotify, if they have queued
data, so lets add a function to chech this.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>