multistart: introduce and use rule_is_initial

* src/gram.h (rule_is_initial): New.
* src/graphviz.c, src/print-xml.c, src/print.c, src/lalr.c: Use it.
Some of these occurrences were incorrect (checking whether this is
rule 0), and not behaving properly in the case of multistart.

src/print-xml.c

@@ -216,8 +216,7 @@ static void
 print_reduction (FILE *out, int level, char const *lookahead,
                  rule *r, bool enabled)
 {
-  const bool final = r->lhs->symbol == acceptsymbol;
-  if (final)
+  if (rule_is_initial (r))
     xml_printf (out, level,
                 "<reduction symbol=\"%s\" rule=\"accept\" enabled=\"%s\"/>",
                 xml_escape (lookahead),