pgsql: Use a 'datallowconn' check for avoiding 'template0', rather than

Log Message:
-----------
Use a 'datallowconn' check for avoiding 'template0', rather than
hardcoding a 'template0' check, per suggestion from Alvaro.

This might fix a problem where someone has allowed 'template0'
connections, but it is a cleaner approach even if doesn't fix the
bug.

Modified Files:
--------------
    pgsql/contrib/pg_upgrade:
        pg_upgrade.c (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_upgrade/pg_upgrade.c?r1=1.2&r2=1.3)