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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270745 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f