Just test that config file is readable

Test that the config file is readable (ie. "-e") before dying, not
strictly a file ("-f").

This allows one to more-easily generate dynamic configuration files
and pass them via `apt-mirror /dev/stdin` rather than having to create
temporary files.

Closes: #824493

apt-mirror

@@ -135,7 +135,7 @@ my %clean_directory = ();
 $config_file = "/etc/apt/mirror.list";    # Default value
 if ( $_ = shift )
 {
-    die("apt-mirror: invalid config file specified") unless -f $_;
+    die("apt-mirror: invalid config file specified") unless -e $_;
     $config_file = $_;
 }