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/discriminator/biased_test.go

@@ -22,10 +22,10 @@ import (
 
 // The biased thresholds.
 const (
-	tagLimitPercent     = "SWW_TAG_LIMIT_PERCENT"
-	asnBytesPercent     = "SWW_ASN_BYTES_PERCENT"
-	countryBytesPercent = "SWW_COUNTRY_BYTES_PERCENT"
-	unknownLimitPercent = "SWW_UNKNOWN_LIMIT_PERCENT"
+	tagLimitPercent     = "SWW_WAF_TAG_LIMIT_PERCENT"
+	asnBytesPercent     = "SWW_WAF_ASN_BYTES_PERCENT"
+	countryBytesPercent = "SWW_WAF_COUNTRY_BYTES_PERCENT"
+	unknownLimitPercent = "SWW_WAF_UNKNOWN_LIMIT_PERCENT"
 )
 
 const (
@@ -34,7 +34,7 @@ const (
 	asnDEHalf     = asnDE + ":50"
 	asnDEQuarter  = asnDE + ":25"
 	countryDEHalf = "de:50"
-	// The same, as SWW_TAG_LIMIT_PERCENT names fromDE's tags.
+	// The same, as SWW_WAF_TAG_LIMIT_PERCENT names fromDE's tags.
 	asnTagDEHalf        = "asn=64496:50"
 	asnTagDEQuarter     = "asn=64496:25"
 	countryTagDEHalf    = "country=DE:50"
@@ -234,7 +234,7 @@ func TestLowestPercentageApplies(t *testing.T) {
 			noCountry, "25 from " + unknownLimitPercent,
 		},
 		{
-			// SWW_UNKNOWN_LIMIT_PERCENT is left at its default, 100.
+			// SWW_WAF_UNKNOWN_LIMIT_PERCENT is left at its default, 100.
 			"the AS number's, for a client without a country",
 			map[string]string{tagLimitPercent: "asn=64500:25"},
 			noCountry, "25 from " + tagLimitPercent,
@@ -497,7 +497,7 @@ func wantPercent(t *testing.T, what string, percent *int64, setting, want string
 }
 
 // percentText gives a limit percentage and the setting that gave it as
-// text, such as "50 from SWW_TAG_LIMIT_PERCENT", or none when both are
+// text, such as "50 from SWW_WAF_TAG_LIMIT_PERCENT", or none when both are
 // left out.
 func percentText(percent *int64, setting string) string {
 	switch {