Return real parent pid in kinfo (used by e.g. ps)

Add a separate field which exports tracer pid and add a new keyword
("tracer") for ps to display it.

This is a follow up to r270444.

Reviewed by:	kib
MFC after:	1 week
Relnotes:	yes
Merge remote-tracking branch 'freebsd/master' into hardened/current/intel-smap