BIRD-LG currently keeps log files indefinitely. This patch allows
setting the number of days logs should be kept for by adding
a `LOG_NUM_DAYS` parameter in the configuration files.
This change is backwards compatible: Running the updated code with
old configuration files will keep logs indefinitely as before; new
deployments based on the example configuration will keep logs
indefinitely.
This simplifies and clarify the configuration, while making it more
generic and getting rid of the "DOMAIN" variable.
Proxied hosts can now be reached over HTTP or HTTPS, through any DNS name,
and even through IP literals to avoid dependency on DNS. With the
previous code, proxied hosts needed to be reachable with the fixed scheme
`http://<host>.<DOMAIN>:<port>`, and this was a big limitation for some
deployments.
This is not backwards compatible, users will have to adapt their
configuration. We rename the `PROXY` variable to `HOSTS` to make that
extra-clear.