Fix for pg_upgrade with extra new cluster databases

The attached, applied patch adds a check to throw a pg_upgrade error
during the check phase, rather than during the post-check upgrade phase.

Specifically, if someone removes the 'postgres' database from the old
cluster and the new cluster has a 'postgres' database, the number of
databases will not match.  We actually could upgrade such a setup, but
it would violate the 1-to-1 mapping of database counts, so we throw an
error instead.

Previously they got an error during the upgrade, and not at the check
stage; PG 9.0.4 does the same.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +