src/config/session.c: Update dbus / systemd activation environment

src/config/session.c: Update dbus / systemd activation environment

This allows xdg-desktop-portal-wlr to work out of the box for screen-recording.
If systemd or dbus is not available the environment update will fail gracefully.

This patch will set XDG_CURRENT_DESKTOP=wlroots but a user may change this by
either having the environment variable set before starting labwc or by having
a different value set in ~/.config/labwc/environment.

Based on PR #461 by @Joshua-Ashton