[krbdev.mit.edu #7005] kdc plugin interfaces need a handle to verto loop

[Sam Hartman via RT]

Currently there is no way other than verto_default
 to get the event loop from a plugin.
This is bad because when we go to libkdc you need to use a particular event loop.
It's important that we add this interface to the plugin API for 1.10 so we don't have backward-incompatible changes when we got to libkdc.