[Nmh-workers] Changes to -attach and -attachformat 1

[Ken Hornstein]

Watching Ronald Guilmette struggle through making the attach command
work, I decided that this stuff should be cleaned up.  I understand
Jon Steinhart's initial reluctance to make attach turned on by
default, but it's been in there for a decade now and I haven't seen
any complaints about it (other than, "How do I make this work again?").

So, in the spirit of improving usability and because I love cramming things
down people's throats whether they like it or not (joke!), I've made
"-attach Nmh-Attachment" & "-attachformat 1" the defaults.  If you already
have set -attach in your .mh_profile, it should continue to work and
use the header name you specify.  I also added a -noattach flag to the
relevant commands so you can turn it off if you desire.

--Ken