ps: add a new option -D to reimplement tree traversal

It takes a non-optional parameter string, one of "up", "down", or "both"
that can request tree traversal in the chosen directions. This adds PIDs
from the paths to the selection of PIDs and can be used together with -d
to draw a subset of the process tree.

Differential Revision: https://reviews.freebsd.org/D41231