library: the <pids> API now supports 'extra' enum sort

library: the <pids> API now supports 'extra' enum sort

The 'PROCPS_PIDS_extra' enumerator already enjoys some
special place wherein it's zeroed with each iteration.

This patch simply extends the special handling to also
include support for sorting. It will be treated as the
'ull_int' data type, since that encompasses the entire
scope of that union within all pids_result structures.

[ plus, we've also corrected the actual 'extra' name ]

This change was prompted by the conversion of ps which
needs that enumerator to store the former 'pcpu' data,
so it will be available for sorting (not for display).

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