#353012 User reference - fix duplicate 'reverse links' on user profile pages when the same user is referenced by several userref fields in a node. Also improves performance by avoiding a full node_load

Fix 'undefined variable' notice when submitting field settings form with 'php code for default value.

Don't assume display_settings[$context] always exists, newly enabled modules may add new contexts that weren't there when the field was last edited.

#336174 (followup) Make sure content_view_field() and content_format() both respect field access rules.

#336174 Move actual field access check in content.module, with content_permission.module providing one permission-based implementation.

Diff integration: refactored around a new hook_content_diff_values() to save contrib field modules the
burden of implementing hook_diff. For most field types, the default content_field_content_diff_values()
should be enough, though.

+ abstract a few element validators

#339537 by markus_petrux: fix orphan fields in {content_group_fields} table when fields are removed.

#331995 Fix 'invalid argument for foreach' warning on nodes with inexistent type. (#327715 followup - content_types() *always* returns a non-array, so testing "if ($type =  content_types($foo))" is useless