kdc: rename windc to kdc plugin

Rename the "windc" plugin API to the more general "kdc" plugin API, for two
reasons: the Heimdal KDC uses the Windows PAC even when not emulating a domain
controller, and the plugin API has accreted methods that are not specific to
emulating a domain controller (such as referral_policy and finalize_reply).

kdc/config.c

@@ -309,7 +309,7 @@ configure(krb5_context context, int argc, char **argv, int *optidx)
 	krb5_enctype_disable(context, ETYPE_DES_PCBC_NONE);
     }
 
-    krb5_kdc_windc_init(context);
+    krb5_kdc_plugin_init(context);
 
     krb5_kdc_pkinit_config(context, config);