epoll exposure

[nia <nia%NetBSD.org@localhost>]

Hey, I regret that epoll was committed without further discussion with
pkgsrc developers. We have a lot of experience with this already
(illumos/SmartOS exposes a compatibility epoll) and the situation is
not entirely great and requires lots of workarounds.

Mostly, the situation of having epoll freely usable in public headers
but discouraged in the man pages doesn't really work. Either we have
barriers against its use, or we encourage its use.

Software is mostly tested on Linux and FreeBSD, which have either epoll
or kqueue. Build systems will detect epoll and use it. Autotools is
falling out of favour, so it is getting harder and harder to override
the detection of operating system functions.  Build systems need to be
patched.

I don't want pkgsrc to be full of "don't use epoll on NetBSD" workaronds.

I think the compatibility support should be opt-in.