reports: don't escape the labels

Currently we use "quotearg" to escape the strings output in Dot.  As a
result, if the user's locale is C for instance, all the non-ASCII are
escaped.  Unfortunately graphviz does not interpret this style of
escaping.

For instance:

    5 -> 2 [style=solid label="\"\303\221\303\271\341\271\203\303\251\342\204\235\303\264\""]

was displayed as a sequence of numbers.  We now output:

    5 -> 2 [style=solid label="\"Ñùṃéℝô\""]

independently of the user's locale.

* src/system.h (obstack_backslash): New.
* src/graphviz.h, src/graphviz.c (escape): Remove, use
obstack_backslash instead.
* src/print-graph.c: Likewise.
* tests/report.at: Adjust.

src/graphviz.h

@@ -63,10 +63,4 @@ void output_red (state const *s, reductions const *reds, FILE *fout);
  */
 void finish_graph (FILE *fout);
 
-/** Escape a lookahead token.
- *
- * \param name         the token.
- */
-char const *escape (char const *name);
-
 #endif /* ! GRAPHVIZ_H_ */