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

schemas/util.sql: mk_diff_query(): parameterized the customizable parts of the query, to allow them to be replaced with their EXPLAIN expansion (which is what we ultimately want, so that the query does not refer to any internal tables or views)

schemas/util.sql: diff(text, text): use util.mk_diff_query(). this splits the complex query-generation code apart from the query-execution code.