[Nmh-workers] mhbuild changes merged to master

[Ken Hornstein]

I've merged my mhbuild changes to master; this includes all of the changes
to make it so mhbuild is always run by send(1) (but in this mode it won't
process mhbuild directives).  This also changed the attach interface
to use the "Attach" header, and will make it people can see what the MIME
headers will look like when using attach.

Speak up if you notice anything amiss.

--Ken