library: extend 'noop' concept to a resettable 'extra'

library: extend 'noop' concept to a resettable 'extra'

The presence of that PROCPS_PIDS_noop may yet see some
use in the future with its 'no alter' library promise.

However, when top used that item to reflect the forest
view nesting level, the unchanging nature of that item
became more of an inconvenience than benefit. For each
refresh top was forced to loop through all the stacks,
resetting that PROCPS_PIDS_noop result struct to zero.

So this commit will now offer users a choice between a
new re-initialized item (PROCPS_PIDS_extra) & the noop
invariant.  Since the library already resets all those
result structures, top will now utilize it at no cost.

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