Updated draft for kadmind plugin API

[Sam Hartman]

The convention in the rest of the code is that everything should take
a krb5_context even if its takes its own context as well.

We can change that convention but it should be a specific choice for
this and future work.

It's probably desirable though that you not have to only call the plugin with the krb5_context used to initialize it.


I'd prefer the plugin to use krb5_set_error_message rather than having
error string returns.

--Sam