RFC: Native epoll syscalls

[Theodore Preduta <theo%pta.gg@localhost>]

While implementing Linux's epoll syscalls in compat_linux for my GSoC
project, my mentor and I had the idea to also add native NetBSD entry
points for epoll.  After some back and forth, we thought it would be
good to solicit opinions from the rest of the community.

There are two main benefits to adding native epoll syscalls:

1. They can be used to help port Linux software to NetBSD.

2. They can be used to test the epoll code with ATF.

It should be noted that this isn't the first time something like this
has been done.  Specifically, benefit 1 has already been used to justify
the existence of clone(2).

So... thoughts?

Theo(dore)