pg_upgrade old cluster delete script

You might remember this pg_upgrade bug report where the user complained
that user-defined tablespaces _inside_ the old cluster directory were
deleted by the old cluster delete script:

    http://www.postgresql.org/message-id/E1THPDm-00018c-KT@wrigleys.postgresql.org

and my reply that we don't want to fix this:

    http://www.postgresql.org/message-id/20121004191201.GA11759@momjian.us

However, creating a script that deletes data is never a good idea so I
have created the attached patch to avoid creating the script in these
cases, and issue a message to the user.

The test isn't perfect but it is probably close enough.  I don't think
it is worth unlinking a possibly matching filename in such cases.

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

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