Activated Jon's attachment system by default and steamlined it.

Activated Jon's attachment system by default and steamlined it.
I had done similar work already some time ago. See:
http://www.mail-archive.com/nmh-workers@nongnu.org/msg02112.html
This time it's the second go.

I changed Jon's attachment system to have a default attachment header
field name, or get it from the profile. I removed the command line options.
The new profile name is: `Attachment-Header'. Stored in attach_fmt.
I removed the the choice between several attachment formats.

A message gets mimeified if it has attachment headers or (and this is new)
contains any non-ASCII character. The body won't be scanned for lines
matching /^#/. The MIME structure is created automatically.

The content-typing of unknown attachments is still poor and needs to
be improved.

You shouldn't use the `mime' command at the whatnow prompt anymore.