* lisp/progmodes/prolog.el: Fix up coding conventions and such.

* lisp/progmodes/prolog.el: Fix up coding conventions and such.
(prolog-indent-width): Use the same default as in
previous prolog.el rather than tab-width which depends on which buffer
is current when the file is loaded.
(prolog-electric-newline-flag): Only enable if electric-indent-mode
is not available.
(prolog-emacs): Remove.  Use (featurep 'xemacs) instead.
(prolog-known-systems): Remove.
(prolog-mode-syntax-table, prolog-inferior-mode-map):
Move initialization into declaration.
(prolog-mode-map): Move initialization into declaration.
Remove system-specific mode-map vars, since they referred to the same
keymap anyway.
(prolog-mode-variables): Obey the user's preference w.r.t
adaptive-fill-mode.  Prefer symbol-value to `eval'.
(prolog-mode-keybindings-edit): Add compatibility bindings.
(prolog-mode): Use define-derived-mode.  Don't handle mercury here.
(mercury-mode-map): New var.
(mercury-mode, prolog-inferior-mode): Use define-derived-mode.
(prolog-ensure-process, prolog-process-insert-string)
(prolog-consult-compile): Use with-current-buffer.
(prolog-guess-fill-prefix): Simplify data flow.
(prolog-replace-in-string): New function to use instead of
replace-in-string.
(prolog-enable-sicstus-sd): Don't abuse `eval'.
(prolog-uncomment-region): Use `uncomment-region' when available.
(prolog-electric-colon, prolog-electric-dash): Use `eolp'.
(prolog-int-to-char, prolog-char-to-int): New functions to use instead
of int-to-char and char-to-int.
(prolog-mode-hook, prolog-inferior-mode-hook): Don't force font-lock.