schemas/util.sql: util.explain2notice_msg(): add newline before and after to visually separate it from other debug info

schemas/util.sql: added mk_set_search_path() (which uses the current search_path)

schemas/util.sql: diff(text, text): in the outputted FULL JOIN query, documented that you should refer to the EXPLAIN output for the expansion of %==

bugfix: schemas/util.sql: first_word(): ignore leading whitespace: need to use util.ltrim_nl(), as ltrim() only removes spaces

bugfix: schemas/util.sql: first_word(): need to ignore leading whitespace. this applies to many of our queries, which have a leading newline.

schemas/util.sql: explain(): don't debug-print the EXPLAIN, to avoid cluttering up the debug output

schemas/util.sql: eval2set(): make debug-printing optional, for use with internal statements

schemas/util.sql: added is_explain()

fix: schemas/util.sql: diff(text, text): FULL JOIN SELECT statement: don't put a comment at the very beginning, because this prevents it from being autoexplained (this is the query we particularly want EXPLAIN output for)

fix: schemas/util.sql: debug_print_sql(): only surround EXPLAIN output with newlines if there is any