[reSIProcate] Problem using extension parameters with Auth

[Alan Stokes <alan@xxxxxxxxxxxxxxxxx>]

Because Auth adds its own overloads for param(), hiding the ones in
ParserCategory, extension parameters don't work quite the way they
should (as specified in
http://warsaw.sjc.purplecomm.com/wiki/index.php?title=Use_Overview#Extension_parameters).

In particular this doesn't compile:
   static const ExtensionParameter p_foo("foo");
   ...
   Auth auth;
   auth.param(p_foo) = "bar";

A workaround is to explicitly qualify:
   auth.ParserCategory::param(p_foo) = "bar";
but this is ugly and means you have to write different code for built-in and extension parameters.

One solution would be to just add a "using ParserCategory::param;" in
the Auth class definition - but that would make all of the usual
parameters available, and I'm not sure that's the intention. An
alternative would be to add to Auth the two overloads
      const Data& param(const ExtensionParameter& param) const;
      Data& param(const ExtensionParameter& param);
and just make both forward to the version in the base class.

- Alan