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.
aba953c3b6