s4:kdc: Rename windc to kdc plugin

s4:kdc: Rename windc to kdc plugin

This is an adaptation to Heimdal:

commit fcff5933ade652343d7c169659da92fac0e6e0d4
Author: Luke Howard <lukeh@padl.com>
Date:   Mon Jan 3 11:10:18 2022 +1100

    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).

NOTE: THIS COMMIT WON'T COMPILE/WORK ON ITS OWN!

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14995

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
(cherry picked from commit 83586e8f5846fff7a8bbe47e743e03166b559584)