Fixed TSort name clash

Without this it's not possible to run portupgrade's commands from bundler
environment, or example:

/usr/src/puppet # bundle exec portversion
```console
/usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkgtsort.rb:35:in `<top (required)>': TSort is not a class (TypeError)
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkgdb.rb:33:in `require'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkgdb.rb:33:in `<top (required)>'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkg.rb:2:in `require'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkg.rb:2:in `<top (required)>'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkgtools.rb:35:in `require'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools/pkgtools.rb:35:in `<top (required)>'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools.rb:1:in `require'
  from /usr/local/lib/ruby/site_ruby/1.9/pkgtools.rb:1:in `<top (required)>'
  from /usr/local/sbin/portversion:38:in `require'
  from /usr/local/sbin/portversion:38:in `<main>'
```
The reason is that there is a name clash between `TSort` class defined in `pkgtsort.rb` and the standard [TSort ruby module](http://ruby-doc.org/stdlib-1.9.3/libdoc/tsort/rdoc/TSort.html)

This patch removes the name clash by renaming `TSort` class to `PkgTSort`.