This change set implements the "context path" discovery for the
CalDav/CardDav endpoints.
This basically implements the bootstrapping process as defined in
RFC6764 section 6, point 2 and 3.
What's missing in this implementation is the fallback that is described
in point 3, subpoint 3, which says that if the context path discovered
in the TXT RR is not reachable the .well-known URI should be used
instead.
I propose to implement this in a future iteration.
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
