library: protect against a future breakage, <pids> ABI

library: protect against a future breakage, <pids> ABI

By co-mingling both external/internal identifiers with
actual implementation code, potential future additions
to our API would have been considerably more difficult.

So, this patch will now rely solely on internal/hidden
identifiers serving as fenceposts in validation logic.

And if the following convention is used for new fields
we will maintain that enumerator alphabetic ordering a
a little longer (even though 2 user fields now don't):

. PROCPS_PIDS_XTRA_FOO_A, PROCPS_PIDS_XTRA_FOO_B, etc.

Reference(s):
http://www.freelists.org/post/procps/me-too-newlib,7

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