[bbagent] Make and signal process group on *nix.
[bbagent] Make and signal process group on *nix.

This should enable bootstrapper-based builds to cancel correctly.

R=chanli, yiwzhang

Change-Id: I786a527a1023c58d79805aaf19fac5d7cd085886
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3340242
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go