Automatically start systemd service if config file is present (#800)

Previously, upgrading the deb package stopped the service but didn't reenable it after a successful upgrade. This made upgrading the package more difficult and required a second step to restart the service. With this commit, the systemd service is automatically started when the default config file is present.

Author: MrSerth

postinst

@@ -7,3 +7,7 @@ systemctl enable pgcat
 if ! id pgcat 2> /dev/null; then
 	useradd -s /usr/bin/false pgcat
 fi
+
+if [ -f /etc/pgcat.toml ]; then
+	systemctl start pgcat
+fi