Initial support for multiple UIs

Move the device discovery code from separate udev helpers to a single
process to listen on two sockets: one SOCK_DGRAM for incoming udev
events, and one SOCK_STREAM for UIs to connect.

Initial support for client/server infrastructure, still need to wire-up
the udev messages.

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