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/discriminator_test.go

@@ -39,7 +39,7 @@ const (
 	// reportBodyLimit is the longest body SPEC.md lets a report have.
 	reportBodyLimit = 1 << 20
 	// anomalyClientPerMinute is the anomaly threshold these tests set.
-	anomalyClientPerMinute = "SWW_ANOMALY_CLIENT_REQUESTS_PER_MINUTE"
+	anomalyClientPerMinute = "SWW_WAF_ANOMALY_CLIENT_REQUESTS_PER_MINUTE"
 )
 
 // The fields of a report, as "discriminator" in SPEC.md names them.
@@ -181,7 +181,7 @@ func (s *Discriminator) history() ratelimit.History {
 }
 
 // metrics returns what the metrics endpoint answers, for a server started with
-// the token as SWW_METRICS_TOKEN.
+// the token as SWW_WAF_METRICS_TOKEN.
 func (s *Discriminator) metrics() string {
 	s.t.Helper()
 
@@ -1485,7 +1485,7 @@ func TestALargeBodyAnnouncedAndLongHeadersAreNoReasonToRefuse(t *testing.T) {
 func TestLogLevelHoldsBackNoRequestLine(t *testing.T) {
 	t.Parallel()
 
-	s := startDiscriminator(t, "", map[string]string{"SWW_LOG_LEVEL": "error"})
+	s := startDiscriminator(t, "", map[string]string{"SWW_WAF_LOG_LEVEL": "error"})
 
 	// A refused question would write a process warning, held back at error.
 	req := s.question(siteA, client, "/")