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/rproxy/limits_test.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/smallwebwaf/internal/config"
+	"sneak.berlin/go/smallwebwaf/internal/config/configtest"
 	"sneak.berlin/go/smallwebwaf/internal/discriminator"
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 	"sneak.berlin/go/smallwebwaf/internal/rproxy"
@@ -33,13 +34,9 @@ func announcedLength(size int) string {
 func TestServerHasTheFixedLimits(t *testing.T) {
 	t.Parallel()
 
-	cfg, err := config.RProxyFromEnvironment(func(name string) (string, bool) {
-		if name == "SWRP_WAF_BACKEND" {
-			return nowhere, true
-		}
-
-		return "", false
-	})
+	cfg, err := config.RProxyFromEnvironment(configtest.Viper(map[string]string{
+		"SWW_RPROXY_WAF_BACKEND": nowhere,
+	}))
 	if err != nil {
 		t.Fatalf("settings: %v", err)
 	}