[PATCH] inet << indexability

This is second take at indexability of << operator for inet types.

Please take a look at it. 

Also, I have a question: I put in a regression test to check that the type
can be indexed, by doing 'explain select ...'. However, the expected
result may vary when the optimizer is tweaked. 

I am not sure if its a good idea to check for that, so feel free to not
commit the regression test part of this patch...If there's a better way to
check that the query will use the index in regression test, I'd like to
know too.

-alex