build: Package sysctl configuration to raise UDP buffer size on Linux (#7417)

* Provide a sysctl config to raise max UDP buffer size

* Add sysctl config to deb

* Check if `deb-systemd-invoke` is available

Co-authored-by: otbutz <tbutz@optitool.de>

script/post-upgrade

@@ -1,3 +1,8 @@
 #!/bin/sh
 
+if command -v deb-systemd-invoke > /dev/null; then
+	deb-systemd-invoke restart procps.service
+else
+	sysctl -q --system
+fi
 pkill -HUP -x syncthing || true