pg_upgrade permission check

I have added the attached patch to pg_upgrade to print a clear error
message if you don't have read/write/execute permission in the current
directory, which is needed for pg_upgrade to read/write temporary files.

This is based on a bug report I received from EnterpriseDB usage
testing.

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

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