[xml/sgml-pkgs] Bug#149013: Add namespace registration mecanism in xmllint --shell.

[Nick Wellnhofer]

This feature was added quite some time ago. The debug shell learned the 
`setns` command in 2004 and the `setrootns` command in 2005. See

https://git.gnome.org/browse/libxml2/commit/?id=bbaa997a06eef7198af749649a3aac4fc6cff0c5
https://git.gnome.org/browse/libxml2/commit/?id=20887eef62c93f1b45ec40f4f4b564730a0ba0e8

Nick