Dreamwidth has dropped challenge auth, since it is incompatible with
secure password handling on the server. (If passwords are hashed in the
database e.g. with bcrypt, the server cannot perform challenge/response
hashing at request time.) Challenge/response has very little other
advantage over clear auth as long as requests are made over TLS, which
should now be the case for all servers.

Details of DW change: https://dw-dev.dreamwidth.org/221167.html

There are some small functional changes:
- comment export gets a progress indicator
- some error reporting is more detailed

'r is sometimes a dict for unknown reasons' is solved:
r was left over from a previous use of the same name (L147-154)