Improve handling of unexpected tracees

Improve handling of unexpected tracees

When receiving a ptrace stop of an unexpected child, handle it
in the most transparent way possible:
- detach it instead of PTRACE_CONT'ing;
- send it the signal with which it has been stopped.
This should hopefully help to deal with processes that have been created
with misused CLONE_PTRACE flag set.

* strace.c (maybe_allocate_tcb) <WIFSTOPPED(status) && !followfork>:
Calculate the signal similarly to the way next_event does,
forward it to the unexpected tracee, and detach the tracee.