add epoll read

[fswang9659]

#6 answer may still go wrong when two or more viface read and pcap capture simultaneously. this patch can solve read problem perfectly

[madscientist42]

Necroing this- but with the immediate return of an EMPTY on the call if it would block...what use is epoll-ing this?

I mean...it only has any value if you're...you know...BLOCKING on the call there. You kind of needed to do Socket type semantics there in this code...because it's the far end of a network stack driver.

[fswang9659]

My test env is, I have created multiple vfiaces, and foreach vifaces, and I want to receive packet if pkt comes from any viface.
so I need a non bolck way to read viface,or I cannot recv from viface，even if anther vfiace has received packet on tun.
Another issue is,as we already know, both two queues on one viface can rx and tx pkt,tx queue is not fixed, depends on selectted by tun driver,I want to epoll two queues on one viface and recv pkt from correct queue.
FYI