schemas/util.sql: materialize_query(): add a comment on the table with the query it was generated from

schemas/util.sql: debug_print_sql(): util.mk_set_search_path(): use for_printing := true to comment out LOCAL

schemas/util.sql: mk_set_search_path(text): include LOCAL commented out, because the user might want to run it with another statement as a single command, in which case it would be useful because they will be in the same transaction (http://www.postgresql.org/docs/9.3/static/sql-set.html#AEN81154)

schemas/util.sql: mk_set_search_path(): added for_printing option like for mk_set_search_path(text)

schemas/util.sql: mk_set_search_path(): support creating a SET search_path statement for display as well, which would exclude LOCAL because it doesn't work as a standalone command (http://www.postgresql.org/docs/9.3/static/sql-set.html#AEN81154)

schemas/util.sql: debug_print_sql(): include the search_path in case the query contains search_path-dependent elements (such as operators)

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