Add $forward_attachments quadoption for inline-forward mode

Add $forward_attachments quadoption for inline-forward mode

When set or answered yes, non text-decodable parts will be attached to
the new message.  The default value is "ask-yes", because I believe
this is something people will want to use, and should be made aware of
the new possible behavior.

The option presents a nice middle ground between previous
inline-forwarding behavior (where all the non-text parts were
dropped), and $mime_forward where the entire email was included as an
attachment.

This was previously difficult to achieve, but after the
recv-attachment refactoring: (a19e5266^..faabd621)
it became possible to use the ATTACH_CONTEXT and the recvattach.c
helper mutt_generate_recvattach_list() to properly deal with nesting
and multiple file-handles.

Co-authored-by: Richard Russon <rich@flatcap.org>