bus: add minimal event loop API

So far we tried to use epoll directly wherever we needed an event loop.
However, that has various shortcomings, such as the inability to handle
larger amounts of timers (since each timerfd costs one fd, which is a
very limited resource, usually bounded to 1024), and inability to do
priorisation between multiple queued events.

Let's add a minimal event loop API around epoll that is suitable for
implementation of our own daemons and maybe one day can become public
API for those who desire it.

This loop is part of libsystemd-bus, but may be used independently of
it.