skeletons: simplify the protections against "unused" warnings.

skeletons: simplify the protections against "unused" warnings.

	* data/c.m4 (b4_parse_param_use): Also accept optional arguments
	to "use".
	Simplify callers.
	* data/glr.c (yyuserAction): Simplify use of b4_parse_param_use.
	(yy_reduce_print): Don't use b4_parse_param_use, as all the arguments
	_are_ used.
	* data/lalr1.cc (YY_SYMBOL_PRINT): Even when disabled, "use" the
	symbol argument.
	This neutralizes a warning in yypush_ when there are no symbols
	with a semantic values.
	(yy_destroy_): Remove useless "use" of yymsg.