vty: Add xsd and a command that can generate the documentation.

When building the doxygen documentation do not remove the other
VTY documentation files in the doc/vty folder. Create a command
that can be installed to dump all nodes and commands as XML on
the given VTY. Create a schema for the XML file and a XSL-T script
that can merge the generated file with additional information.
diff --git a/Makefile.am b/Makefile.am
index d0fb027..c9b7ccd 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -36,8 +36,7 @@
 	$(DOXYGEN) Doxyfile.gsm
 
 $(top_builddir)/doc/vty/html/index.html: $(SOURCES) Doxyfile.vty
-	@rm -rf doc/vty
-	mkdir -p doc/vty
+	@rm -rf doc/vty/{html,latex}
 	$(DOXYGEN) Doxyfile.vty
 
 $(top_builddir)/doc/codec/html/index.html: $(SOURCES) Doxyfile.codec