Add DSSSL mode and share code with newly required

Add DSSSL mode and share code with newly required
lisp-mode as much as possible.
(scheme-mode-variables): Copy filling stuff from lisp-mode.
Add an outline-regexp.
(scheme-mode-map): Inherit shared-lisp-mode-map and provide a menu.
(dsssl-sgml-declaration): New variable.
(dsssl-mode): New command.
(dsssl-font-lock-keywords): New variable.
(scheme-indent-function): Near copy of lisp-indent-function.
(scheme-comment-indent, scheme-indent-offset,
scheme-indent-function, scheme-indent-line,
calculate-scheme-indent, scheme-indent-specform,
scheme-indent-defform, scheme-indent-sexp): Removed; use lisp-mode
equivalents.
(scheme-imenu-generic-expression): New variable.
(dsssl-imenu-generic-expression): New variable.
(scheme-let-indent): Use lisp-indent-specform.