service: hook up custom endpoint logic

If BusPolicy= was passed, the parser function will have created
an ExecContext->bus_endpoint object, along with policy information.

In that case, create a kdbus endpoint, and pass its path name to the
namespace logic, to it will be mounted over the actual 'bus' node.

At endpoint creation time, no policy is updloaded. That is done after
fork(), through a separate call. This is necessary because we don't
know the real uid of the process earlier than that.