bs.el: Janitorial work; most importantly use `special-mode`

bs.el: Janitorial work; most importantly use `special-mode`

* lisp/bs.el: Prefer # to quote function arguments.
(bs-mode-font-lock-keywords): Use backquote; quote face names; and use
a list of faces instead of two applications at the same spot.
(bs-sort-buffer-interns-are-last, bs-config--files-and-scratch)
(bs-configurations, bs--intern-show-never): Fix ^$-vs-\`\' confusion.
(bs-mode-map): Remove bindings made redundant by inheritance.
(bs--redisplay): Use `line-number-at-pos`.
(bs--goto-current-buffer): Use `regexp-opt`.
(bs-mode): Inherit from `special-mode`.
(bs--current-buffer, bs--up): Use `point-min`.
(bs--create-header-line): Remove redundant arg.

(cherry picked from commit bb62e435637c7422741189384fa89e2272caec5b)