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)

internal/waf/waf_test.go

@@ -15,7 +15,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/waf"
 )
 
-// defaultDisabledRules are the rules SWW_WAF_DISABLED_RULES switches off
+// defaultDisabledRules are the rules SWW_WAF_CORE_RULE_SET_DISABLED_RULES switches off
 // by default.
 //
 //nolint:gochecknoglobals // a constant cannot be a list
@@ -525,7 +525,7 @@ func giteaRequests(t *testing.T) []request {
 func TestOrdinaryGiteaRequestsPassAndAttacksAreRefused(t *testing.T) {
 	t.Parallel()
 
-	const threshold = 5 // SWW_WAF_ANOMALY_THRESHOLD's default
+	const threshold = 5 // SWW_WAF_CORE_RULE_SET_ANOMALY_THRESHOLD's default
 
 	crs := atDefaults(t)
 
@@ -550,7 +550,7 @@ func TestOrdinaryGiteaRequestsPassAndAttacksAreRefused(t *testing.T) {
 	}
 }
 
-// bodyLimit is SWW_WAF_BODY_LIMIT in the tests that read bodies.
+// bodyLimit is SWW_WAF_CORE_RULE_SET_BODY_LIMIT in the tests that read bodies.
 const bodyLimit = 8 << 10
 
 // The Content-Type headers of the kinds of body the Core Rule Set reads.