dbus-monitor: handle default case for binary mode header

dbus-monitor: handle default case for binary mode header

Also comment why it's OK to not do anything for the modes that don't
have a header. We are effectively treating the default case as one
of those, on the assumption that future modes are more likely to
lack a header than to have one.

Based on part of a patch from Thomas Zimmermann.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98191