[development] block visibility by roles (#18018) -- code needs review (again)

[Derek Wright]

since dries specifically mentioned this feature in his talk today on  
what he wants to see in core for the next release, i figured i should  
update the patch. ;)

http://drupal.org/node/18018#comment-102128
http://drupal.org/files/issues/block_roles_6.patch

please review and test.

i'd like to provide some kind of 4.7 work-around, but i can't promise  
i'll have time to work on that in the near future.  let's get the  
functionality committed to core, and then we can worry about a 4.7  
contrib solution.

thanks!
-derek (dww)