discover-client: interact directly with waitset

Currently, clients need to mess with the discover client fd directly,
and manually register the waiter.

Instead, this change adds a waitset parameter to
discover_client_register, so that the discover client can register
itself, and call discover_client_process directly. This means no proxy
handlers, and no casts to waiter callbacks.

We can also get rid of discover_client_get_fd.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>