Add aggsortop column to pg_aggregate, so that MIN/MAX optimization can

Add aggsortop column to pg_aggregate, so that MIN/MAX optimization can
be supported for all datatypes.  Add CREATE AGGREGATE and pg_dump support
too.  Add specialized min/max aggregates for bpchar, instead of depending
on text's min/max, because otherwise the possible use of bpchar indexes
cannot be recognized.
initdb forced because of catalog changes.