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