One common method for CalDAV or CardDAV clients to find the current user
principal URL is to request the `/.well-known` URL (see [RFC 6764,
section 6][1]), expecting a redirect. Such URL is for example a valid
result of the discovery phase described in that RFC. The expectation is
that a client, given such URL, is able to find the principal URL by
following a redirect when sending a PROPFIND request.
This change makes `PropfindFlat()` (and, by extension,
`FindCurrentUserPrincipal()`) handle such a redirect and correctly
return the requested properties, even if their HREF is different from
the original request path.
[1]: https://datatracker.ietf.org/doc/html/rfc6764#section-6
03633121d9