Read every setting through viper and rename the settings (closes #211)

Every setting of discriminator and rproxy is read through viper, in place
of the os.LookupEnv function Main passed down. NewViper turns AutomaticEnv
and AllowEmptyEnv on, so a setting that is set but empty reads as set. A
setting may also be given as a file, the _FILE form, which rproxy's
settings now have too; rproxy still writes nothing to disk.

The settings are renamed. Discriminator's take the prefix SWW_WAF_,
rproxy's take SWW_RPROXY_, and the shared SWW_COMMAND keeps SWW_. The Core
Rule Set's settings take SWW_WAF_CORE_RULE_SET_ rather than SWW_WAF_WAF_.
Every place a name appeared changed: the code, the viper keys, tests,
documents, examples, the README's settings table and the image files.

Model: deepseek-v4-pro

docs/metrics.md

@@ -29,7 +29,7 @@ keeps the `instance` label as `exported_instance` unless the scrape sets
 
 ## Countries and AS numbers
 
-For the `SWW_METRICS_TOP_N` busiest of each, the rest as `other`:
+For the `SWW_WAF_METRICS_TOP_N` busiest of each, the rest as `other`:
 
 - `sww_country_requests_total`, `sww_country_request_bytes_total`,
   `sww_country_response_bytes_total` and `sww_country_list_refusals_total`, by