lib: ZeroMQ read handler, v2

lib: ZeroMQ read handler, v2

This uses zmq_getsockopt(ZMQ_FD) to create a libfrr read event, which
then wraps zmq_poll and calls an user-specified ZeroMQ read handler.
It's wrapped in a separate library in order to make ZeroMQ support an
installation-time option instead of build-time.

Extended to support per-message and per-fragment callbacks as discussed
with Bingen in PR #566.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>