procutil: make recent fix for zombies compatible with py2

The fix in ed684a82e29b (procutil: always waiting on child processes
to prevent zombies with 'hg serve', 2020-05-07) works only on Python 3
because it passes a daemon argument to threading.Thread(). Python
2 requires you to assign to the .daemon property instead. Python 3
also seems to support that, so this patch fixes the code by
unconditionally using the old form.

Differential Revision: https://phab.mercurial-scm.org/D8657