variables: accept dashes.

* 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.
(cherry picked from commit 4f646c3794)

Conflicts:

	data/bison.m4
	src/parse-gram.y

src/parse-gram.y

@@ -547,15 +547,12 @@ rhs:
 
 variable:
   ID
-  | STRING { $$ = uniqstr_new ($1); } /* deprecated and not M4-friendly */
-  ;
+| STRING { $$ = uniqstr_new ($1); }
+;
 
 /* Some content or empty by default. */
 content.opt:
-  /* Nothing. */
-    {
-      $$ = "";
-    }
+  /* Nothing. */   { $$ = ""; }
 | STRING
 ;