Added check in send(1) of attach (default Nmh-Attachment) headers to

ensure that only plain files are attached.  Otherwise, it is a fatal
error.  Note that whatnow's attach will continue to allow attachment
of directories because it expands those out to their contents.  It
doesn't check what the contents are, though.  That's why we needed to
add this check.

Also added a check to skip attach headers that are empty so that
they can be put in drafts, per Norm's request.