Introduce a new comment style "c" flag.

Introduce a new comment style "c" flag.
* src/syntax.c (SYNTAX_FLAGS_COMMENT_STYLEB)
(SYNTAX_FLAGS_COMMENT_STYLEC): New macros.
(SYNTAX_FLAGS_COMMENT_STYLE): Use them, add an argument.
(syntax_prefix_flag_p): New function.
(Fstring_to_syntax): Understand new "c" flag.
(Finternal_describe_syntax_value): Recognize new flag; use the
SYNTAX_FLAGS_* macros.
(scan_sexps_forward, Fparse_partial_sexp): Change representation of
comment style to accomodate the new styles.
(back_comment, forw_comment, Fforward_comment, scan_lists)
(scan_sexps_forward): Update code to obey the new comment style flag.
* src/syntax.h: Move SYNTAX_FLAGS_FOO() macros to syntax.c.
* src/casefiddle.c (casify_region): Use the new syntax_prefix_flag_p.
* lisp/progmodes/octave-mod.el (octave-mode-syntax-table): Use the new "c"
comment style.
* lisp/progmodes/scheme.el (scheme-mode-syntax-table):
* lisp/emacs-lisp/lisp-mode.el (lisp-mode-syntax-table): Remove spurious
"b" flag in "14b" syntax.

Fixes: debbugs:6834