role: dbus

This adds support for the integrating libdbus into the lws event loop.

Unlike the other roles, lws doesn't completely adopt the fd and libdbus insists
to retain control over the fd lifecycle.  However libdbus provides apis for
foreign code (lws) to provide event loop services to libdbus for the fd.

Accordingly, unlike the other roles rx and writeable are not subsumed into
lws callback messages and the events remain the property of libdbus.

A context struct wrapper is provided that is available in the libdbus
callbacks to bridge between the lws and dbus worlds, along with
a minimal example dbus client and server.