webdav: Fix return code on DELETE of absent file

Due to compatibility concerns with MS Office 2010, Milton in some
cases turns a 404 reply into 401. Atlas noticed this and complained.

Milton only does this for anonymous requests, however when using client
certificates, from the point of view of Milton the session is anonymous
as no HTTP level authentication was performed.

This patch adds an X509AuthenticationHandler for Milton. It doesn't
actually do any authentication, but it is sufficient to let Milton
discover that the request has been authenticated and return the
correct error code.

Target: trunk
Require-notes: yes
Require-book: no
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7614/