chore: import upstream snapshot with attribution

systemd/install_check.pl

@@ -0,0 +1,19 @@
+#!/usr/local/bin/perl
+# Decides whether the standalone systemd module should be shown.
+
+use strict;
+use warnings;
+use lib "..";
+
+use WebminCore;
+
+# is_installed(mode)
+# Returns Webmin's install-check code for systems with systemctl available.
+sub is_installed
+{
+# Mode 0 is a boolean probe; mode 1 asks whether the module should be visible.
+return 0 if (!has_command("systemctl"));
+return $_[0] ? 2 : 1;
+}
+
+1;