run: Make D-Bus session bus default to $XDG_RUNTIME_DIR/bus

This matches what's now implemented in libdbus, GDBus and sd-bus.
If $DBUS_SESSION_BUS_ADDRESS is unset, check whether $XDG_RUNTIME_DIR/bus
is a socket owned by Flatpak's uid; if it is, use it.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #2874
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f