Rename system test directory with common files to _common

The old name "common" clashes with the convention of system test
directory naming. It appears as a system test directory, but it only
contains helper files.

To reduce confusion and to allow automatic detection of issues with
possibly missing test files, rename the helper directory to "_common".
The leading underscore indicates the directory is different and the its
name can no longer be confused with regular system test directories.

bin/tests/system/stop.pl

@@ -189,7 +189,7 @@ sub stop_rndc {
 	my $how = $halt ? "halt" : "stop";
 
 	# Ugly, but should work.
-	system("$RNDC -c ../common/rndc.conf -s $ip -p $port $how | sed 's/^/I:$test:$server /'");
+	system("$RNDC -c ../_common/rndc.conf -s $ip -p $port $how | sed 's/^/I:$test:$server /'");
 	return;
 }