edbus: new epoll-dbus-integration helper

This helper can be used to integrate a dbus connection into any existing
event-loop. It uses epoll to multiplex all the dbus sources into a
single FD.

Note that this helper only provides a sane basic dbus integration. All
the insane features like thread-local event-loops, remote thread wakeup,
signal-handling, ... are not handled by the helper. This really doesn't
belong into the dbus library so we don't support it. Stop using this
crap and start using basic file-descriptor multiplexing exclusively!

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>