guivm: clean qrexec-service related environment in GUI session

Xephyr server and the GUI (xfce) session running inside is started now
as a qrexec service. Do not let qrexec-related environment variables to
propagate to the actual session, to not confuse some utilities.
This specifically affects qvm-sync-appmenus which behaves differently
when called as a qrexec service. But could affect also
qrexec-fork-server (a service could see gui-session related variables if
for any reason qrexec-fork-server fails to set them to the
actual call values).

appvm-scripts/usrbin/qubes-start-xephyr

@@ -5,6 +5,15 @@ if [ $(whoami) == "root" ]; then
     exit 1
 fi
 
+# do not let child processes believe they are running as qrexec service
+# directly
+unset QREXEC_REMOTE_DOMAIN
+unset QREXEC_SERVICE_FULL_NAME
+unset QREXEC_SERVICE_ARGUMENT
+unset QREXEC_REQUESTED_TARGET
+unset QREXEC_REQUESTED_TARGET_TYPE
+unset QREXEC_AGENT_PID
+
 DISPLAY_XORG=:1
 DISPLAY_XEPHYR=:0