schemas/util.sql: diff_any(): documented how to run EXPLAIN on the FULL JOIN query

schemas/util.sql: removed non-useful diff_queries() (it is not possible to match distinct `record` types to a common return type, so this would not have worked). use diff_views() instead.

bugfix: schemas/util.sql: diff_queries(), diff_views(): embed views verbatim as FROM items, so that the type of their corresponding columns is the view's rowtype. this is necessary in order to match the types of the columns to the return type (set by col_type_null).

schemas/util.sql: diff_cols(): renamed to diff_queries() because multiple columns are supported

fix: schemas/util.sql: diff_cols(): don't assume that each side will have one column, as many of our validations queries have multiple columns

bugfix: schemas/util.sql: diff_views(): need to use $2, not $1, for the 2nd query

fix: schemas/util.sql: materialize_query(): remove any extra blank line before the query, to improve readability when newlines are used as a log message separator and are therefore confusing in the middle of a log message

schemas/util.sql: added ltrim_nl()

bugfix: schemas/vegbien.sql: setting search_path locally: also need to set function option search_path, in order to limit the effects of `SET LOCAL search_path` (mk_set_search_path()) to the current function (http://www.postgresql.org/docs/9.1/static/sql-set.html#AEN75423)

bugfix: schemas/vegbien.sql: setting search_path locally: need to use `SET LOCAL search_path TO` instead of `PERFORM set_config(search_path, ..., is_local := true)` because the meaning of is_local refers to the entire transaction, not just the current function (as for LOCAL)