Add an enable/disable setting for HTTP authentication

[Pierre-Marie de Rodat]

For debugging purposes, it would be nice to be able to disable HTTP authentication (prologin.timeauth), so that making requests using curl would be easy again.

To be defined: would this setting be global (/etc/prologin/timeauth.yml) or local to each daemon/client?

[Antoine Pietri]

Added in 8e51a50.