top: utilize a library result struct for 'forest view'

top: utilize a library result struct for 'forest view'

When top was originally adapted to use that <pids> API,
the forest view support was redesigned since the proc_t
pad_3 byte could no longer be employed to hold a task's
nesting level. The redesign required additional arrays.

Now that the dust is settling on those initial efforts,
that PROCPS_PIDS_noop item was used as a substitute for
the old pad_3 along with a return to the former design.
But, while it proved adequate, the invariant nature for
that item required of top an extra initialization step.

So the library was coaxed into adding one more pid_item
(PROCPS_PIDS_extra) which will, unlike that 'noop' guy,
be reset with each reap. Everybody should be happy now.

Signed-off-by: Jim Warner <james.warner@comcast.net>