cck 6.x-2.1

This release mainly fixes two critical issues:
- #331293 by Timo.Kissing - Content copy: no fields proposed for export (#320632 followup).
- #331033 Views integration: Fix fatal error in content_handler_field.inc in some circumstances.

Other fixes:
- #331179 Userreference: 'reverse link' checkbox stayed unchecked.
- Uninstall forgot to remove some variables.
- #331794 Fix false positives for "The default value is invalid" error message.
- #331995 Fix 'invalid argument for foreach' warning on nodes with inexistent type.
- Views integration: make sure our own render_link doesn't output empty links (see #332679).