util: fix the kdbus_verify_uid_prefix() to check if the kuid is mapped

The user that operates on the domain must have a mapping into the domain
userns and must provide the matching uid to be allowed to create buses
and endpoints.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
endpoint: push a comment about custom endpoints name validation

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
changelog: note the bus and endpoint name validation fix

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>