postal greeting values don't allow legitimate duplicates

when defining a postal greeting option (as well as email and addressee), the interface rejects a record if the label matches an existing value. but there are legitimate cases where you would have the same label applied to different contact types – for example, both individual and household could have a dear nick name option.

see: http://forum.civicrm.org/index.php?topic=15523

the easier way to handle this would be to alter the formRule in: CRM/Admin/Form/OptionValue.php
so that it either ignores the rule when on the greetings forms, or adds a condition based on contact type – where a matching label + matching contact type is rejected