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/.gitignore b/.gitignore
index 3c8afd9..b91b5aa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -61,7 +61,8 @@
 
 doc/codec
 doc/core
-doc/vty
+doc/vty/latex
+doc/vty/html
 doc/gsm
 doc/html.tar