Add ptrace(2) reporting for LWP events.
Add two new LWPINFO flags: PL_FLAG_BORN and PL_FLAG_EXITED for reporting
thread creation and destruction. Newly created threads will stop to report
PL_FLAG_BORN before returning to userland and exiting threads will stop to
report PL_FLAG_EXIT before exiting completely. Both of these events are
only enabled and reported if PT_LWP_EVENTS is enabled on a process.
Call kern_thr_exit() instead of duplicating it.
This was already missing the racct_subr() call from kern_thr_exit(), but
this will now also add TDB_EXIT ptrace events for threads that exit this