Add epoll-based nonblocking file I/O
Add epoll-based nonblocking file I/O

It's useful to be able to do nonblocking concurrency based on sockets
and files, especially in a control/management plane context.  It's also
useful to test Fibers before the TCP implementation is fully ported.

Igalia/snabb,eugeneia/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch,Igalia/snabb,SnabbCo/snabbswitch,SnabbCo/snabbswitch,alexandergall/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,Igalia/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,snabbco/snabb,Igalia/snabb,eugeneia/snabb,SnabbCo/snabbswitch,snabbco/snabb,eugeneia/snabb,Igalia/snabb,Igalia/snabbswitch,snabbco/snabb,snabbco/snabb,Igalia/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,snabbco/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,alexandergall/snabbswitch,Igalia/snabb,snabbco/snabb,snabbco/snabb,Igalia/snabb,eugeneia/snabb,Igalia/snabb,Igalia/snabbswitch,alexandergall/snabbswitch,eugeneia/snabb,Igalia/snabb