On bus startup check given auth in config file against supported mechanisms.

On bus startup check given auth in config file against supported mechanisms.

With recent code starting dbus-daemon with an unsupported auth mechanism
let dbus-daemon silently ignore this issue. Clients connecting to this
server fails to connect without any descriptive explanation of the
root cause, only the message 'Rejected client connection due to lack
of memory' error is reported in dbus-daemon verbose log, which is disabled
in production environments.

With this patch dbus-daemon checks the supported auth mechanisms on startup
and shuts down with a descriptive error message, which gives admins an
immediate feedback on service startup/restart.

Signed-off-by: Ralf Habacker <ralf.habacker@freenet.de>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99622