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_.

Model: deepseek-v4-pro (first version); opus-5-5 (rework)

docs/alerts.md

@@ -77,8 +77,8 @@ on one line:
 }
 ```
 
-- `instance`: `SWW_INSTANCE_NAME`. `time`: when the alert was raised. `event`:
-  one of the events above.
+- `instance`: `SWW_WAF_INSTANCE_NAME`. `time`: when the alert was raised.
+  `event`: one of the events above.
 - `host`: the site of the request that raised the alert, since one
   `discriminator` serves many sites. It is left out of `source_failure`,
   `file_error` and `summary`.
@@ -122,8 +122,8 @@ Slack gets the title in bold and the text below it, with `&`, `<` and `>`
 escaped. ntfy gets the text as the message, with these headers:
 
 - `Title`: the title;
-- `Authorization`: the token, as `Bearer <token>`, while `SWW_ALERT_NTFY_TOKEN`
-  is set;
+- `Authorization`: the token, as `Bearer <token>`, while
+  `SWW_WAF_ALERT_NTFY_TOKEN` is set;
 - `Priority` and `Tags`: by event, as listed below.
 
 The priority and tag by event: