Use 'qrexec-client-vm --use-stdin-socket' to properly handle EOF

When qrexec-client-vm is called from a systemd unit connected to a
socket, it the same socket on both stdin and stdout. Tell
qrexec-client-vm about it, so it can use shutdown() instead of close()
to properly deliver EOF. It will also make the qrexec-client-vm to use
just stdin FD.

QubesOS/qubes-issues#9169

vm-systemd/qubes-updates-proxy-forwarder@.service

@@ -2,6 +2,6 @@
 Description=Forward connection to updates proxy over Qubes RPC
 
 [Service]
-ExecStart=/usr/bin/qrexec-client-vm '' qubes.UpdatesProxy
+ExecStart=/usr/bin/qrexec-client-vm --use-stdin-socket '' qubes.UpdatesProxy
 StandardInput=socket
 StandardOutput=inherit