bus: use inline trace argument for ANONYMOUS auth

bus: use inline trace argument for ANONYMOUS auth

Rather than using a separate DATA round to transmit the trace-string of
the ANONYMOUS authentication scheme, transmit it inline as argument.
This requires a refactor of the client-side SASL parser, as we now have
a different set of replies depending on the mode used.

This fixes an issue where libdbus-1 does not query for trace-strings if
not transmit inline as AUTH-ANONYMOUS argument. It is unclear from the
wording of the spec whether this is a violation by libdbus-1. However,
we can work around it by simply changing our mode of transmittal.