When cloning template0 (or other fully-frozen databases), set the new

database's datallowconn and datfrozenxid to the current transaction ID
instead of copying the source database's values.  This is OK because we
assume the source DB contains no normal transaction IDs whatsoever.
This keeps VACUUM from immediately starting to complain about unvacuumed
databases in the situation where we are more than 2 billion transactions
out from the XID stamp of template0.  Per discussion with Milen Radev
(although his complaint turned out to be due to something else, but the
problem is real anyway).