read/write of new WebDAV properties

The new WebDAV properties specified in Section 4 should all specify whether
they are read/write, or read-only.  For example, DAV:comment is read/write,
and DAV:supported-method-set, DAV:supported-live-property-set, and
DAV:supported-report-set are all obviously read only.
DAV:creator-displayname is less obvious.  I think it should be read/write,
especially if it is being advertised as being generally applicable across
all servers, but there certainly will be cases where it wouldn't be
writeable.

- Jim