- invoke command using shell option `exec` which then replaces the
  current process with the newly forked subprocess

  This shortens the `ps` output footprint, as the invoker script, eg.
  `xpcli` will no longer appear there. Also this makes process control
  work where other processes start a runner, remember its PID and later
  send a KILL signal to that PID; before this change, it just killed
  the runner process, leaving the underlying `php` process alive.
Merge pull request #11 from xp-framework/fix-kill

Good to merge.