`taskkill /T /F` only starts terminating the tree, and the caller then
waits for its direct child. A grandchild could still be exiting, with its
files open, when `kill_tree` returned or `Peer::exited` resolved. The
descendants are now opened before the kill and waited for (up to 5 s)
after it. A process counts as a descendant only if it started after its
parent, so a reused parent id doesn't pull in unrelated processes.
Closes#1270
l0ng-ai
7fd330912a