Fix support for except in nameclasses

For https://bugzilla.gnome.org/show_bug.cgi?id=565219

The code was imply missing even if simple, added a few regression
tests.

relaxng.c

@@ -3819,7 +3819,11 @@ xmlRelaxNGCompareNameClasses(xmlRelaxNGDefinePtr def1,
             return (0);
         return (1);
     } else if (def1->type == XML_RELAXNG_EXCEPT) {
-        TODO ret = 0;
+        ret = xmlRelaxNGCompareNameClasses(def1->content, def2);
+	if (ret == 0)
+	    ret = 1;
+	else if (ret == 1)
+	    ret = 0;
     } else {
         TODO ret = 0;
     }

result/relaxng/565219_0.err

+./test/relaxng/565219_0.xml:1: element foo: Relax-NG validity error : Element foo has wrong namespace: expecting http://bar.com/
+./test/relaxng/565219_0.xml fails to validate

result/relaxng/565219_1.err

+./test/relaxng/565219_1.xml validates

result/relaxng/565219_2.err

+./test/relaxng/565219_2.xml:1: element foo: Relax-NG validity error : Element foo has wrong namespace: expecting http://bar.com/
+./test/relaxng/565219_2.xml fails to validate

result/relaxng/565219_err

+./test/relaxng/565219.rng validates

test/relaxng/565219.rng

+<grammar xmlns="http://relaxng.org/ns/structure/1.0">
+  <start>
+   <element>
+      <anyName>
+        <except>
+          <nsName ns="http://bar.com/"/>
+          <nsName ns="http://foo.com/"/>
+        </except>
+      </anyName>
+      <empty/>
+   </element>
+  </start>
+</grammar>
+

test/relaxng/565219_0.xml

+<foo xmlns="http://foo.com/"/>

test/relaxng/565219_1.xml

+<foo/>

test/relaxng/565219_2.xml

+<foo xmlns="http://foo.com/"/>