MFC 292894,292896: Add ptrace(2) reporting for LWP events.

MFC 292894,292896: Add ptrace(2) reporting for LWP events.

292894:
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.

292896:
Document the recently added support for ptrace(2) LWP events.

git-svn-id: svn://svn.freebsd.org/base/stable/10@304017 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f