Use ptrace(2) LWP events to track threads reliably in truss.

Use ptrace(2) LWP events to track threads reliably in truss.

- truss can now log the system call invoked by a thread during a
  voluntary process exit.  No return value is logged, but the value passed
  to exit() is included in the trace output.  Arguments passed to thread
  exit system calls such as thr_exit() are not logged as voluntary thread
  exits cannot be distinguished from involuntary thread exits during a
  system call.
- New events are now reported for thread births and exits similar to the
  recently added events for new child processes when following forks.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D5561