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/system.h

@@ -171,6 +171,23 @@ typedef size_t uintptr_t;
 # define obstack_sgrow(Obs, Str) \
   obstack_grow (Obs, Str, strlen (Str))
 
+/* Output Str escaped to be a string.
+
+   For instance "\"foo\"" -> "\\\"foo\\\"".  */
+
+# define obstack_backslash(Obs, Str)                    \
+  do {                                                  \
+    char const *p__;                                    \
+    for (p__ = Str; *p__; p__++)                        \
+      switch (*p__)                                     \
+        {                                               \
+        case '"':  obstack_sgrow (Obs, "\\\""); break;  \
+        case '\\': obstack_sgrow (Obs, "\\\\"); break;  \
+        default:   obstack_1grow (Obs, *p__);   break;  \
+        }                                               \
+  } while (0)
+
+
 /* Output Str escaped for our postprocessing (i.e., escape M4 special
    characters).