use poll instead of select

use poll instead of select

Particularly when using the go-lxc api with lots of threads, it
happens that if the open files limit is > 1024, we will try to
select on fd > 1024 which breaks on glibc.

So use poll instead of select.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>