kdc: KDC plugin API contract notes

Add some notes about the KDC plugin API contract, and require plugins to
explicitly indicate which version of the API they support (remove the macro
alias for the current version).

kdc/kdc-plugin.c

@@ -49,7 +49,7 @@ static const char *kdc_plugin_deps[] = {
 static struct heim_plugin_data kdc_plugin_data = {
     "krb5",
     "kdc",
-    KRB5_KDC_PLUGIN_MINOR,
+    KRB5_PLUGIN_KDC_VERSION_8,
     kdc_plugin_deps,
     kdc_get_instance
 };