[PATCH] auto-enable kqueue support

This bumps kqueue from experimental to fully available net I/O method.

kqueue has been in use on FreeBSD and maybe others for some time now and
has less bugs than epoll. So the issues on record should not be held
against it.

The attached patch adds auto-detection for the kqueue dependencies and
enables it by default when it can build. Unfortunately due to the
dependencies we cannot add it to maximus layer for force-enable, but the
default layer will test it on FreeBSD at least. It is still less
preferred than epoll(), but more than select() and poll().

Amos