Enable KDC TCP listener by default

Make the KDC default to listening on TCP.

(cherry picked from commit 8d88e2ab00be126237569dc72827ced2ce6b7d04)

ticket: 6731
Document KDC TCP listener change

Update documentation to reflect the change in the default KDC TCP
listener behavior, new in 1.13.

(cherry picked from commit 59cbb7662282f6f882b5d108cf45bdd042857c6a)

ticket: 6731
version_fixed: 1.13
status: resolved