LDAP attribute mapping

[Arran Cudbard-Bell]

> 
>> If rlm_rest and rlm_cache have attribute models that are elegant and well thought out then let's move everything to that model. On the other hand if ulang is conceptually cleaner then lets move rlm_rest and rlm_cache to a ulang solution. Pick one idea and make everything follow those rules.
> 
> With 2.0 there was an effort to maintain configuration compatibility, which limited standardisation efforts. Maintaining config compatiblity with 3.0 we don't have that constraint.

* Were not maintaining config compatibility with 3.0 so we don't have that constraint.

Just talking with Alan, apparently whole subsections can be referenced. This is especially useful for things like TLS, where you're using the same certs in multiple places, and you can define a global TLS section and then reference it from the various module configuration.

-Arran