When the process stared by strace exits/terminates, forget its pid

When the process stared by strace exits/terminates, forget its pid

The process stared by strace can exit/terminate only once.  When this
happens, forget its pid, so that any other event or action related to a
process with the same pid as the pid of that first process would not be
confused with that first process.

* strace.c (print_signalled, print_exited): Clear strace_child when it
equals to the given pid.