session/logind: check for XDG_SESSION_ID first
session/logind: check for XDG_SESSION_ID first

In order to support compositors running as systemd user units without display manager,
a mechanism for specifying session ID exactly must exist.

Checking for `XDG_SESSION_ID` mimics loginctl behaviour https://github.com/systemd/systemd/blob/e95be7def26c6c5feaf08a4135aa4f50c53263a8/src/login/loginctl.c#L856.

ascent12/wlroots,swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots