Use epoll-shim to emulate epoll(7) on FreeBSD

FreeBSD does not provide epoll(7) and instead requires an external library,
epoll-shim, that implements epoll() using kqueue(2)

Co-authored-by: Jan Beich <jbeich@FreeBSD.org>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>