Display-based word wrapping

[Chong Yidong]

I have been studying Kim Storm's display-based word wrapping code.  I
believe it's pretty safe, and suitable for inclusion into the Emacs 23
release (probably turned off by default).

The original patch included a `wrap-column' variable, but I'd like to
modify this to use `truncate-lines' instead, as shown below.  This would
mean that word wrap always uses the right window edge as a wrap column.
Any objections?



truncate-lines is a variable defined in `C source code'.

Documentation:
Whether lines longer than the window width are truncated.
If the value is nil or `word-wrap', long lines are displayed using
continuation lines.  If nil, the wrapping occurs at the right window
edge.  If `word-wrap', it occurs at the rightmost space or tab
character, if possible.

Any other non-nil value means to give each line of text just one
screen line, truncating long lines at the right window edge.

Note that this is overridden by the variable
`truncate-partial-width-windows' if that variable is non-nil
and this buffer is not full-frame width.