Support system_bus_socket to be found in /var/run/dbus as well as /run/dbus.

Currently the path to system_bus_socket is hardcoded to
  /run/dbus/system_bus_socket
which works everywhere for systemd.

However, distributions which do no symlink /var/run on /run will have
it only accessible via
  /run/dbus/system_bus_socket
which should be supported by elogind, too.

Closes #77

Signed-off-by: Christoph Willing <chris.willing@linux.com>
Sigend-off-by: Sven Eden <sven.eden@prydeworx.com>