Disallow unexpected replies

Disallow unexpected replies

Allowing to send replies when NO_REPLY_EXPECTED is set is useless in
practice: Clients need to be careful not to send these replies, because
bus policy could deny these messages. The spec even mentions that this
issue exists.

To make this more clear and misbehaving clients less likely, disallow
sending unexpected replies entirely.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75749
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>