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/bytelimits_test.go

@@ -19,13 +19,13 @@ import (
 
 // The byte limit settings.
 const (
-	bytesLimitPerMinute = "SWW_BYTES_LIMIT_PER_MINUTE"
-	bytesLimitPerHour   = "SWW_BYTES_LIMIT_PER_HOUR"
-	bytesLimitPerDay    = "SWW_BYTES_LIMIT_PER_DAY"
-	bytesCount          = "SWW_BYTES_COUNT"
+	bytesLimitPerMinute = "SWW_WAF_BYTES_LIMIT_PER_MINUTE"
+	bytesLimitPerHour   = "SWW_WAF_BYTES_LIMIT_PER_HOUR"
+	bytesLimitPerDay    = "SWW_WAF_BYTES_LIMIT_PER_DAY"
+	bytesCount          = "SWW_WAF_BYTES_COUNT"
 )
 
-// The values of SWW_BYTES_COUNT.
+// The values of SWW_WAF_BYTES_COUNT.
 const (
 	countResponse = "response"
 	countRequest  = "request"
@@ -40,7 +40,7 @@ const (
 	// byteLimit is the byte limit these tests set, as a setting: a request
 	// with a body and its answer, 100 bytes, go over it.
 	byteLimit = "99"
-	// minuteBytes is limit_hit for SWW_BYTES_LIMIT_PER_MINUTE.
+	// minuteBytes is limit_hit for SWW_WAF_BYTES_LIMIT_PER_MINUTE.
 	minuteBytes = "minute_bytes"
 )
 
@@ -227,8 +227,8 @@ func TestByteLimitsLeaveOutWhatTheRateLimitsLeaveOut(t *testing.T) {
 	t.Parallel()
 
 	const (
-		allowed = "192.0.2.7"  // in SWW_ALLOW_NETS
-		exempt  = "192.0.2.10" // in SWW_RATE_LIMIT_EXEMPT_NETS
+		allowed = "192.0.2.7"  // in SWW_WAF_ALLOW_NETS
+		exempt  = "192.0.2.10" // in SWW_WAF_RATE_LIMIT_EXEMPT_NETS
 	)
 
 	s, _ := startWithAnswers(t, map[string]string{