assoc_type and assoc_handle

[Brad Fitzpatrick]

I noticed that checkid_* mode is defined like:

    * GET OpenID server URL
    * openid.mode = 'checkid_immediate'
    * openid.identity = OpenID URL
    * openid.return_to = return URL
    * openid.assoc_handle = HMAC secret handle (optional, see "dumb mode")

But no assoc_type.

So if in the future somebody does associate and request UMAC or HMAC-Tiger
or whatever, and the server accepts, there's no way to actually use that
assoc_handle you got with the checkid_* modes.

So we need to either add assoc_type to checkid_* mode (easier?) or merge
assoc_type into the assoc_handle, as was proposed with the x.500/ldap/etc
format of "{HMAC-TIGER}gibberishfooo"

I'll assume the addition of assoc_type to checkid_* modes for now.

- Brad