dot/emacs, el/dot-emacs.el: Fix `split-window-horizontally'.

dot/emacs, el/dot-emacs.el: Fix `split-window-horizontally'.

The standard version takes the space used for fringes and scroll bars
out of the space you request for the left window.  (But, oddly, if you
give a negative width, for the right window, then you get what you asked
for.)  This is annoying, so fix it.

Actually fixing (or even advising) `split-window-horizontally' is too
risky to compatibility, so I've invented a new function
`mdw-split-window-horizontally' which does the job right, and rebound
C-x 3 to use the new version.