safe_process puts its children (mysqld, in this case) into a separate
process group, to be able to kill it all at once.

buildslave kills mtr's process group when it loses connection to
the master.

result? buildslave kills mtr and safe_process, but leaves stale
mysqld processes in their own process groups.

fix: put safe_process itself into a separate process group, then
buildslave won't kill it and safe_process will kill mysqld'd
and itself when it will notice that the parent mtr no longer exists.