Further work on information_schema. _pg_keyissubset() wasn't quite

fully search-path-proof yet; also, element_types view did not work for
parameters and result types of functions, because it didn't generate
the object_name for the function the same way the data_type_privileges
view does.  While at it, centralize dependencies on INDEX_MAX_KEYS/
FUNC_MAX_ARGS into a function returning setof int, so that it will be
easier to fix information_schema for nonstandard values of these
parameters.