Disable state messages for attachments when forwarding.
First, option(OPTVIEWATTACH) is only set for (s->flags & DISPLAY), so that
check can be ignored.
With that removed, all this is doing is printing
[-- This is an attachment ...]
with no context in the middle of non-display rendering for the case
$honor_disposition is set with a handler.
This makes no sense and I believe is a logic error. Perhaps this
never took effect, but with $forward_attachments it now affects
forwarding in that case.