Unfold multi-line headers during composing.

Parse multi-line headers properly(?). Convert them into single lines
as the staging screen and text entry cannot handle multi-line strings.
Squeeze linear-whitespace when unfolding header field bodies.
Fold long header lines when sending message.

Only "References:" is treated specially as a "structured" header field.
All other header fields are treated as "unstructured" for now.
Support text entry wider than screen.
Remove hack that stops drawing one column early.

I don't remember the reason for it.  Possibly I had not configured
curses, and so it would move to the next line automatically.
text_entry: Avoid invalid argument to isprint().

The invalid argument can cause a segmentation fault with glibc.