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.