Allow CCK noderef and user relationships to work nicely with "multiple"

This patch creates a content_handler_relationship that will allow the views administrator to select which delta to use (or all) when multiples are allowed. It defaults to All so existing functionality will not change, but this allows some interesting flexibility.

One use case: Let's say I've set up a noderef with a maximum of 3 multiples. I could set up the same relationship 3 times using different delta values to get each node all in the same row.