shadow: start-on-last-client-exit does not execute when exit-with-client is set

[stdedos]

With:

Xpra-Client-Python3-x86_64_2.5-r22135\xpra_cmd" shadow ssh://user@ip/0  --clipboard=no --opengl=no --desktop-scaling=0.75 --webcam=no --speaker=off --microphone=off

commands

start-on-last-client-exit=bash -c 'touch /run/user/1000/xpra-test`date +%s`'
start-on-last-client-exit=bash -c '`date`'
start-on-last-client-exit=bash -c '. /run/user/1000/dbus-session; gnome-screensaver-command -l'

are not executed, because, implicitly exit-with-client=yes is set (xpra-client remotely starting xpra-server)

---

Fix the code so that, start-on(-last)-client-exit=command is always executed, even if the server will be terminated through any means.

ticket:2128#comment:4 onwards.