Add declarations for new KDC authdata interface

Add a pluggable interface for KDC authdata using the current plugin
infrastructure, mirroring the KDC functionality of authdata_plugin.h.
Like the old interface, this one isn't yet public.
Use new KDC authdata interface in kdc_authdata.c

Remove the server authdata interface declarations from authdata.h and
the code to load and invoke old KDC authdata modules.  Add code to
load and invoke authdata modules using the new kdcauthdata pluggable
interface.
Modernize kdc_authdata.c

Adjust whitespace, identifier names, and in some cases flow control.
No functional changes.
Update test KDC authdata module to new interface

Remove plugins/authdata/greet, which was a v0 KDC module.  Modify
plugins/authdata/greet_server to use the new interface.  Within
greet_auth.c, remove the unused function greet_kdc_verify.  Build the
greet_client and greet_server modules by default, but do not install
them.
Add KDC authdata tests

Add a new test script t_authdata.py and a C harness adata.c to test
KDC authdata handling logic.  KDB module authdata is not currently
tested.