bugfix: lib/sh/db.sh: pg_schema_exists(): need to ignore benign error exit status from the "cannot create temporary relation in non-temporary schema" error

lib/sh/util.sh: stderr_matches(): supporting ignoring any benign error exit status associated with the error message being tested for

lib/sh/util.sh: stderr_matches(): usage: documented where any ignore_e statement would go

bugfix: lib/sh/util.sh: stderr_matches(): can't use `try` because this clears the exit status, which is needed for @PIPESTATUS to work. to support this, also need to avoid errexiting since @PIPESTATUS will be used instead.

lib/sh/util.sh: added dp(), which debug-prints a message

lib/sh/db.sh: psql(): $verbose_ok: renamed to $bypass_ok for clarity, because this applies only to the `--output /dev/fd/41` bypass (which when not possible, requires turning off verbose output

fix: lib/sh/db.sh: psql(): added $output_data switch analogous to what mysql() has. this causes query results of eg. void-returning functions to be correctly filtered by the logging mechanism, rather than output to stdout.

fix: lib/sh/db.sh: psql(): verbosity=0 (errors only) mode: use `SET client_min_messages = WARNING;` instead of NOTICE to hide verbose messages within psql as well

lib/sh/db.sh: psql(): replaced `test "$verbose_ok" && can_log` with bool var $verbose_

fix: lib/sh/db.sh: psql(): $verbose_: renamed to $verbose_ok for clarity