Create a signal-safe type for tracking spawned pids

Create a signal-safe type for tracking spawned pids

* util.cxx (spawned_pids_t): New type which wraps a set<pid_t>, masking
  signals on each access to ensure consistency in and out of the signal
  handler.  The spawned_pids global is the only instance.
  (stap_waitpid): Use !contains(pid) rather than count(pid)==0.
  (kill_stap_spawn): Use spawned_pids_t::killall().