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

internal/discriminator/admin.go

@@ -51,7 +51,7 @@ func (rq *request) answerHealth() {
 
 // answerAdmin answers a request for one of discriminator's own endpoints. Each
 // needs a token, sent as Authorization: Bearer <token>: the metrics
-// SWW_METRICS_TOKEN, the others SWW_ADMIN_TOKEN. A request without it is
+// SWW_WAF_METRICS_TOKEN, the others SWW_WAF_ADMIN_TOKEN. A request without it is
 // refused with 401. An endpoint whose token is unset answers 404, as any other
 // path does.
 func (rq *request) answerAdmin() {