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