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.
checkpoint