lib/sh/util.sh: override the `.` builtin (a.k.a. `source`) and run canon_rel_path on the included script. this removes .. in the path when it's displayed in bash error messages.

lib/sh/util.sh: log_err(): ensure errors are visible by using 2>&22. since this is uses log(), verbosity=1 also needs to be set.

lib/sh/util.sh: $explicit_errors_only: explicitly set verbosity=0 to hide startup logging

lib/sh/util.sh: limit_stderr(): if stdout goes to stderr (i.e. is logging info), assume that stderr_is_errors

lib/sh/util.sh: $explicit_errors_only: documented that this option should only be used for testing the explicit error displaying, as otherwise, important output may be missed

lib/sh/util.sh: $explicit_errors_only: added usage

bugfix: lib/sh/util.sh: command(): updated $errors_only var name to $explicit_errors_only

lib/sh/util.sh: $explicit_errors_only: noted that verbosity=0 displays everything that isn't explicitly hidden

lib/sh/local.sh: psql(): use new stderr_is_errors=1 since psql's logging output comes on stdout, so stderr contains only errors. (the caveat of this is that the query output is normally mixed with logging info on stdout, and needs to be separated out to a different file descriptor using --output.)

lib/sh/util.sh: added $explicit_errors_only env var which shows only explicitly-displayed errors (those which have been redirected to fd 22). note that most of the time, this has the same effect as `verbosity=0 script...`.