Force a new process group for our cleanup pathways to avoid SIGKILL.
Force a new process group for our cleanup pathways to avoid SIGKILL.

Do this since buildbot now likes to blast the full process group,
and we have processes that explicitly break out of our process group
(I'm looking at you mr. vmtest kvm process).

Additionally, ensure stdin/stdout/stderr are flushed if at all
possible.

BUG=None
TEST=kill -9'ing cbuildbot.  Oddly therapeutic.

Change-Id: Ie443b4cd48aea6ced2b46be7876c163734b4e366
Reviewed-on: https://gerrit.chromium.org/gerrit/40431
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,coreos/chromite,zhang0137/chromite