x/build/cmd/buildlet: detect and kill stray processes

[bradfitz]

The buildlet, especially in reverse buildlet mode, needs to learn how to kill stray processes leftover from previous builds.

(This isn't relevant for VM and container-based builders, since the VM or container kill everything when they exit)

But on OS X and other weirder builders where we run the buildlet in reverse-dialing mode, I sometimes see stray processes. Like this:

Notice testgo spinning:

(I think @ianlancetaylor fixed that bug, but still.... we should be tolerant of such bugs in the future)

This is a tracking bug to make the buildlet smarter: enumerate processes at start and end, and kill new things which didn't go away on their own, especially if they're owned or parented by us. Maybe scan the process list often during builds to learn parents, so we know if they were ours or not.

/cc @zombiezen @adg

[bradfitz]

/cc @dmitshur