Cleanup: Refactor libauth Config object(s)

Refactoring the Auth::Config objects to get v5 building again.

The object storing auth_param settings is renamed to SchemeConfig.

A new Auth::Config object is added. The objects holding auth_param,
auth_schemes and other authenticate_* directives settings are stored
there instead of just auth_params.

Lots of outdated doxygen docs that have more up to date copies elsewhere
are removed. The "\ingroup AuthAPI" are left for now since some auth
things are not yet in the Auth:: namespace, but other auth related
groups are dropped completely.

Code parsing and dumping auth_param lines has been de-duplicated. Along
with the keep_alive and utf8 settings, which are shared by multiple
schemes. This allows dropping several virtual methods from NTLM and
Negotiate scheme objects.

An auth/forward.h file is added for predefines. Some initial Auth::
symbols are listed there. It is not complete, but others can be added as
needed later.

Some unnecessary includes have been removed. There are probably more. I
just did the obvious ones related to the new auth/forward.h and
auth/Config.h files at this time.