Make Emacs column width configurable.

This change introduces a new metaconfiguration variable `emacs-width'
which defaults for 77 for the usual historical reasons.  It's used:

  * to set the default `loose' Emacs frame width, in `.Xdefaults', via
    some fiddly C preprocessor hacking;

  * to choose the initial Emacs frame size, in `.xinitrc';

  * to decide how to divvy up windows, refill text, highlight overlong
    lines, and all of that good stuff, in `.emacs' and `dot-emacs.el'.

Emacs itself now has two new variable, `mdw-column-width' which is the
`physical' column width, used to decide how big to make windows and
frames; and `mdw-text-width' which is a potentially buffer-local
variable holding the logical text width in the buffer, used for
refilling text and highlighting long lines.

This is probably not completely right yet, but it seems like a good
start for supporting environments where I'm expected to use a different
column width most of the time.

If `emacs-width' is unset, or left as its default, there should be no
change.