dot-emacs: Fix divvy-window for Emacs 21.

In Emacs 22 (and since I last looked) fringe-columns has become a
defsubst (i.e., it's inline-expandable), and its expansion refers to
yet another function not available in Emacs 21.  Since I want to use the
byte-compiled result with Emacs 21 too, we engage in some chicanery to
hide the symbol names from the byte compiler.

Also make the interactive version accept a prefix argument controlling
the column width (on the grounds that the feature was already there
anyway, but not for some reason available).