schemas/util.sql: added prepend_comment()

schemas/util.sql: removed no longer used mk_set_search_path(VARIADIC schemas text[]). use mk_search_path() instead.

schemas/util.sql, vegbien.sql: comment about "function option search_path": don't include `mk_set_search_path()` since that is no longer used

schemas/util.sql: diff(text, text): always use the schema of col_type_null (the common base type) as the search_path, since any custom %== operator for it will always be in the same schema as it

schemas/util.sql: use_own_schema(): renamed to use_schema() because this can be used for any type-linked schema

bugfix: rematerialize_out_view(): set_search_path() does not take the same arguments as mk_set_search_path() did, so need to use mk_search_path() instead

schemas/util.sql: use_own_schema(): auto-append util to the search_path to enable use of util operators

schemas/util.sql: mk_set_search_path(): no need to debug_print_return_value() anymore because functions now use set_search_path() (or something that calls it), which debug-prints the statement (`EXECUTE util.mk_set_search_path()` did not)

schemas/util.sql: mk_search_path(), mk_set_search_path(text...): auto-append util to the search_path to enable use of util operators

schemas/util.sql: removed no longer used mk_use_own_schema(). use util.use_own_schema() instead.