mtr: use env for perl

On FreeBSD, perl isn't in /usr/bin, its in /usr/local/bin or
elsewhere in the path.

Like storage/{maria/unittest/,}ma_test_* , we use /usr/bin/env to
find perl and run it.

scripts/convert-debug-for-diff.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/perl -i
+#!/usr/bin/env perl -i
 #
 # This script converts all numbers that look like addresses or memory sizes,
 # in a debug files generated by --debug (like mysqld --debug-dbug), to #.