tinydm-run-session.sh: support starting dbus session (MR 15)

If a tinydm session is unable to start a dbus session automatically, e.g.
i3wm, support starting one if configured in the profile. This is useful to run
programs that require a dbus session bus, e.g. feedbackd under such desktop
environments.

It can be enabled by creating a TINYDM_START_DBUS_SESSION environment variable
in $HOME/.profile, /etc/profile, or, better still, a file under
/etc/tinydm.d/env-x11.d/ . The value must be set to "true".

Reference: https://wiki.gentoo.org/wiki/D-Bus#The_session_bus

tinydm-run-session.sh

@@ -91,6 +91,16 @@ run_session() {
 		export XDG_CURRENT_DESKTOP="$desktop_names"
 	fi
 
+	# Check if profile or session profile requests tinydm to start a dbus
+	# session. This is indicated by exporting the value "true" in a
+	# TINYDM_START_DBUS_SESSION environment variable in either profile
+	if [ -z "$DBUS_SESSION_BUS_ADDRESS" ] &&
+	   [ "$TINYDM_START_DBUS_SESSION" = "true" ] &&
+	   [ "$(command -v dbus-launch)" ]; then
+		cmd=$(command -v "$cmd")
+		cmd="$(command -v dbus-launch) --exit-with-x11 $cmd"
+	fi
+
 	echo "--- tinydm ---"
 	echo "Date:    $(date)"
 	echo "Session: $resolved"