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.

schemas/util.sql, vegbien.sql: use util.use_own_schema()/util.set_search_path() instead of EXECUTE util.mk_use_own_schema()/util.mk_set_search_path()

schemas/util.sql: added use_own_schema()

schemas/util.sql: added set_search_path()

schemas/util.sql: runnable_sql(): don't output search_path before a SET statement

schemas/util.sql: added is_set_stmt()

schemas/util.sql: diff(text, text): use mk_diff_query()'s new cols param to avoid a nested SELECT