(prolog-program-name): Use gprolog if available.

(prolog-program-name): Use gprolog if available.
(prolog-mode-syntax-table, prolog-mode-abbrev-table, prolog-mode-map):
Bring together declaration and initialization.
(prolog-mode-variables): Don't set the syntax table.
Don't set paragraph-start and comment-indent-function.
Add /*..*/ to the comment regexps.
(prolog-mode-commands): Remove.  Do it during init of prolog-mode-map.
(prolog-mode-map): Don't bind TAB.
(prolog-mode): Set the syntax table.
(prolog-comment-indent): Remove.
(inferior-prolog-mode-map): Initialize in the declaration.
(inferior-prolog-mode-syntax-table)
(inferior-prolog-mode-abbrev-table): New vars.
(inferior-prolog-mode): Derive from comint-mode.
(run-prolog): Avoid switch-to-buffer which can fail in dedicated and
minibuffer windows.