XML: upgrade-2.10.xsl: rsc instance_attrs [2/2]: deal with pcmk_arg_map

See commit 6c8e0be20.

Also make MapMsg (MapMsg-2) "function" and cibtr-2.rng accommodate
the new value selector (@in-case-of-droppable-prefix) encoded in the
upgrade "map".

Also further refine allowed
{primitive|template)/instance_attributes/nvpair/@name
to
those beside the just-dropped (possibly because of metamorphosis into
the another one) items as encoded in the upgrade map.