Fix strange %define locations for default values.

Reported by Akim Demaille at
<http://lists.gnu.org/archive/html/bug-bison/2007-12/msg00001.html>
and discussed again starting at
<http://lists.gnu.org/archive/html/bison-patches/2008-11/msg00102.html>.
* data/bison.m4 (b4_percent_define_default): Leave syncline blank
because location information is bogus.
Use angle brackets to delimit fake file name because square brackets
look like over-quoted m4.  Choose a better fake file name.
Use negative line numbers.
* src/muscle_tab.c (muscle_percent_define_default): Likewise.
* src/location.c (location_print): If line for a boundary is negative,
only print that boundary's file name.
* src/location.h: Document that.
* tests/skeletons.at (%define Boolean variables: invalid skeleton
defaults): Update output.

data/bison.m4

@@ -1,8 +1,8 @@
                                                             -*- Autoconf -*-
 
 # Language-independent M4 Macros for Bison.
-# Copyright (C) 2002, 2004, 2005, 2006, 2007, 2008 Free Software Foundation,
-# Inc.
+# Copyright (C) 2002, 2004, 2005, 2006, 2007, 2008, 2009 Free Software
+# Foundation, Inc.
 
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -464,10 +464,9 @@ m4_define([b4_percent_define_default],
 [m4_ifndef([b4_percent_define(]$1[)],
            [m4_define([b4_percent_define(]$1[)], [$2])dnl
             m4_define([b4_percent_define_loc(]$1[)],
-                      [[[[[Bison:b4_percent_define_default]:1.0]], [[[Bison:b4_percent_define_default]:1.0]]]])dnl
-            m4_define([b4_percent_define_syncline(]$1[)],
-                      [[]b4_syncline(1, [["[Bison:b4_percent_define_default]"]])[
-]])])])
+                      [[[[<skeleton default value>:-1.-1]],
+                        [[<skeleton default value>:-1.-1]]]])dnl
+            m4_define([b4_percent_define_syncline(]$1[)], [[]])])])
 
 # b4_percent_define_check_values(VALUES)
 # --------------------------------------