Do not allow identifiers that start with a dash.

This cleans up our previous fixes for a bug whereby Bison
discarded `.field' in `$-1.field'.  The previous fixes were less
restrictive about where a dash could appear in an identifier, but
the restrictions were hard to explain.  That bug was reported and
this final fix was originally suggested by Paul Hilfinger.  This
also fixes a remaining bug reported by Paul Eggert whereby Bison
parses `%token ID -123' as `%token ID - 123' and handles `-' as an
identifier.  Now, `-' cannot be an identifier.  Discussed in
threads beginning at
<http://lists.gnu.org/archive/html/bug-bison/2011-01/msg00000.html>,
<http://lists.gnu.org/archive/html/bug-bison/2011-01/msg00004.html>.
* NEWS (2.5): Update entry describing the dash extension to
grammar symbol names.  Also, move that entry before the named
references entry because the latter mentions the former.
* doc/bison.texinfo (Symbol): Update documentation for symbol
names.  As suggested by Paul Eggert, mention the effect of periods
and dashes on named references.
(Decl Summary): Update documentation for unquoted %define values,
which, as a side effect, can no longer start with dashes either.
* src/scan-code.l (id): Implement.
* src/scan-gram.l (id): Implement.
* tests/actions.at (Exotic Dollars): Extend test group to exercise
bug reported by Paul Hilfinger.
* tests/input.at (Symbols): Update test group, and extend to
exercise bug reported by Paul Eggert.
* tests/named-refs.at (Stray symbols in brackets): Update test
group.
($ or @ followed by . or -): Likewise.
* tests/regression.at (Invalid inputs): Likewise.

tests/actions.at

@@ -158,6 +158,52 @@ AT_PARSER_CHECK([./input], 0,
 [[15
 ]])
 
+# Make sure that fields after $n or $-n are parsed correctly.  At one
+# point while implementing dashes in symbol names, we were dropping
+# fields after $-n.
+AT_DATA_GRAMMAR([[input.y]],
+[[
+%{
+# include <stdio.h>
+  static int yylex (void);
+  static void yyerror (char const *msg);
+  typedef struct { int val; } stype;
+# define YYSTYPE stype
+%}
+
+%%
+start: one two { $$.val = $1.val + $2.val; } sum ;
+one: { $$.val = 1; } ;
+two: { $$.val = 2; } ;
+sum: { printf ("%d\n", $0.val + $-1.val + $-2.val); } ;
+
+%%
+
+static int
+yylex (void)
+{
+  return 0;
+}
+
+static void
+yyerror (char const *msg)
+{
+  fprintf (stderr, "%s\n", msg);
+}
+
+int
+main (void)
+{
+  return yyparse ();
+}
+]])
+
+AT_BISON_CHECK([[-o input.c input.y]])
+AT_COMPILE([[input]])
+AT_PARSER_CHECK([[./input]], [[0]],
+[[6
+]])
+
 AT_CLEANUP