Various functions are smarter about using the correct windows
and the appropriate keymaps.
(vcursor-string): New variable, shows vcursor on dumb terminals.
(vcursor-map): New variable, holds keymap for vcursor commands.
(vcursor-use-vcursor-map): New variable, non-nil if vcursor
keys are overlaid onto main keymap.
(vcursor-toggle-vcursor-map): New function to implement this.
(vcursor-interpret-input): New variable, copy input as if typed.
(vcursor-window-funcall): With list instead of symbol, now calls interactively.
(vcursor-isearch-backward): New function to match forward isearch.
