Trivial changes to silence warnings.

Trivial changes to silence warnings.
(xscheme-previous-mode, xscheme-previous-process-state): Add defvars.
(xscheme-last-input-end, xscheme-process-command-line, xscheme-process-name,
xscheme-buffer-name, xscheme-expressions-ring-max, xscheme-expressions-ring,
xscheme-expressions-ring-yank-pointer, xscheme-running-p,
xscheme-control-g-synchronization-p, xscheme-control-g-disabled-p,
xscheme-string-receiver, default-xscheme-runlight, xscheme-runlight,
xscheme-runlight-string, xscheme-process-filter-state, xscheme-allow-output-p,
xscheme-prompt, xscheme-mode-string): Move to beginning of file.
(scheme-interaction-mode-commands-alist, scheme-interaction-mode-map,
scheme-debugger-mode-map): Declare them before use.  Note: the initialization
code for the variables has not been moved because it uses functions that
reference the variables.
(xscheme-control-g-message-string, xscheme-process-filter-alist,
xscheme-prompt-for-expression-map): Declare them before use.
(scheme-debugger-mode-commands): "?\ " -> "?\s".