* lib/rubygems/commands/push_command.rb:  Fixed credential download for
  `gem push --host`
* lib/rubygems/gemcutter_utilities.rb:  ditto.
* test/rubygems/test_gem_commands_push_command.rb:  Test for the above.
* test/rubygems/test_gem_gemcutter_utilities.rb:  ditto.

* lib/rubygems/config_file.rb:  Abort if the `gem push` credentials
  file has insecure permissions.
* test/rubygems/test_gem_config_file.rb:  Test for the above.

* lib/rubygems/ext/builder.rb:  Do not look for Gemfile, Isolate, etc.
  while building gem extensions.

* lib/rubygems/package.rb:  Unset spec and files list if a gem's
  signatures cannot be verified.
* test/rubygems/test_gem_package.rb:  Test for the above.

* lib/rubygems/specification.rb:  Reduce use of eval.
* lib/rubygems/test_case.rb:  ditto.

* test/rubygems/test_gem_specification.rb:  Test setting
  specification_version for legacy gems.  Dup Gem.ruby before
  untainting in case it's frozen.

* lib/rubygems.rb:  Reduce use of eval.  Only read files when looking
  for Gemfile, Isolate, etc.
* test/rubygems/test_gem.rb:  Test for the above.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39055 b2dd03c8-39d4-4d8f-98ff-823fe69b080e