Add a KillProcess function

We cannot depend on all environments to have a start-stop-daemon or
similar tool. We instead implement a KillProcess function that behaves
similar to “start-stop-daemon --retry”.

Note that the attached unittest can hang in foreground if the child
misbehaves (doesn't write to the internal pipe). Since unittest are
either run in the foreground or are run with a timeout from an automated
framework, I think this is an acceptable trade-off (against of using
hardcoded timeouts in the test).

Reviewed-by: imsnah