bug#869: Bug in handling invisible text, and bug in Pmail.

[Richard M. Stallman]

Rmail hides some headers by making a copy of the whole header.
In that copy it deletes the uninteresting headers.
Babyl format makes special provision for that.
Pmail cannot use that method.

If we make Pmail always display the current message decoded in a
separate buffer, we could delete uninteresting header fields from that
buffer.