Please document the available third-party KDC modules.

http://mailman.mit.edu/pipermail/krbdev/2019-April/013098.html mentions FreeIPA and Samba. 

For Samba, reference https://wiki.samba.org/index.php/Roadmap_MIT_KDC .

https://web.mit.edu/kerberos/krb5-1.17/ says KDC supports S4U2Self requests when used with the Samba KDB module, but 
https://wiki.samba.org/index.php/Roadmap#Active_Directory_Server leaves the impression S4U2Self does not work with
Samba.  Please clarify.