Port 'Ability to select primary key' patch to D7

Hi,

I've had a go at porting the patch here to D7:
https://www.drupal.org/node/887550

I've got it working with User entity fields, but not currently with content_profile (Profile2?) - for which i've included an empty stub for the hook_user_import_primary_key_check() implementation.

By and large this is a like-for-like patch - please take a look and consider rolling into a full release.

cheers