Fix up startChild to be useful for tests
Fix up startChild to be useful for tests

Now it will wait for the first apparently terminal signal, trace that,
and cause the tracee to exit. This lets us use it to get the same
backtrace we'd expect from a core file, without needing the core file.

peadar/pstack,peadar/pstack,peadar/pstack,peadar/pstack