must be used for the new database, except when copying from template0.
This is the same rule that we now enforce for locale settings, and it has
the same motivation: databases other than template0 might contain data that
would be invalid according to a different setting.  This represents another
step in a continuing process of locking down ways in which encoding violations
could occur inside the backend.  Per discussion of a few days ago.

In passing, fix pre-existing breakage of mbregress.sh, and fix up a couple
of ereport() calls in dbcommands.c that failed to specify sqlstate codes.