[krbdev.mit.edu #8265] Test suite can be affected by installed authdata, locate plugin modules

[Greg Hudson via RT]

The locate and authdata pluggable interfaces are older than the plugin 
framework code, and does not honor plugin_base_dir or any other profile 
variable, so there is no way to prevent test programs from loading and 
being affected by locate plugin modules in the install tree.

The authdata interface should be converted to use the new plugin 
framework, which we can do without any backwards compatibility since it 
is not public.  (It might even be reasonable to eliminate it.)  The 
locate interface is public, so needs to be treated more carefully.