webdav: Fix problems with root paths

Add support for enforcing a user-specific root directory in the WebDAV door.
Attempts to read or write to a directory outside the user's root directory
will result in a permission denied error being reported back to the user.
    
As a special case, if the user has a user-specific root and that user
attempts to read '/' then the reply is a redirection to that user's
home directory (if specified) or their root directory if not.
    
The patch also fixes the CSS and adds the missing reference to the favicon.
    
Require-book: yes
Require-notes: yes
Signed-off-by: Dmitry Litvintsev
Signed-off-by: Gerd Behrmann
Patch: http://rb.dcache.org/r/5201/
Patch: http://rb.dcache.org/r/5672/