Return user basepath for current-user-principal

Fix #196

The way to do collection discovery described in
http://stackoverflow.com/a/11673483 doesn't work well with Radicale:

1. current-user-principal returns /user/calendar.ics/

2. PROPFINDs asking for calendar-home-set will return the URL that was
   used in the request, which is still /user/calendar.ics/

3. The final PROPFIND with Depth: 1 is supposed to list all collections,
   but because the request is done with a collection URL, Radicale returns
   the items for the "calendar.ics" collection which might or might not
   exist.