Child processes aren't auto-killed reliably #3

The adapter currently relies on pydevd to kill child processes. This doesn't work so well when the debuggee process is force-killed, as in e.g. test_autokill. Worse yet, when the IDE requests "terminate", the adapter will force-kill the process itself, preventing the logic for children from kicking in.

The adapter needs to perform its own cleanup for child processes when a "launch" session exits.