`git-annex addurl --fast/--relaxed --verifiable` now uses VURL keys,
which is an improvement over the old, un-verifiable URL keys. But users
have to know to use it, and can have URL keys in their repository.

Note that old git-annex, when in a repo with VURL keys, is still able to
operate on them fine, even though it doesn't know what they are.
Only fsck warns about them. So --verifiable could become the default
reasonably soon. It's not necessary to wait for everyone to have the new
version of git-annex.

It might be good to nudge users to migrate their existing files to VURL
eventually. Could be a fsck warning about URL keys, at some point after
--verifiable becomes the default for addurl. Or could be a warning when
transferring the content between repositories that it's not possible to
verify it.

Of course if users want to continue to use their existing URL keys and 
not be able to verify content, that's fine. Users can also choose to use 
WORM keys if they really want to. 

However, I don't think there's enough reason to want to use URL keys to add
configuration of which kind of keys addurl uses, once VURL is the default.
--[[Joey]]

> One way this might cause trouble is that current `git-annex registerurl`
> and `unregisterurl` (and `fromkey`)  when passed an url rather than a key,
> generates an URL key. If that is changed to generate a VURL key, then
> it might break some workflow, particularly one where an url was
> registered as an URL key and is now being unregistered.
