variables: accept dashes.

	* data/bison.m4 (b4_percent_define_if_define_): Also map dashes to
	underscores.
	* src/scan-gram.l ({id}): Also accept dashes after the initial
	letter.
	({directive}): Use {id}.
	* src/parse-gram.y: Comment and formatting changes.
	* doc/bison.texinfo (Symbols): Adjust the lexical definitions of
	symbols.
	* src/complain.h, src/complain.c (yacc_at): New.
	* src/symtab.c (symbol_new): Use yacc_at to report inappropriate
	symbol names.
	* src/output.c (token_definitions_output): Do not #define token
	names with dashes.