Add PHP7.3 support and let setup.php write initial config files

etc/systemd/system/php7.3-fpm.service

@@ -0,0 +1,17 @@
+# This service is actually a systemd target,
+# but we are using a service since targets cannot be reloaded.
+
+[Unit]
+Description=The PHP 7.3 FastCGI Process Manager (multi instance master)
+Documentation=man:php-fpm7.3(8)
+After=network.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/bin/true
+ExecReload=/bin/true
+
+[Install]
+WantedBy=multi-user.target
+