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/state/state_test.go

@@ -931,7 +931,7 @@ func TestUnwritableDirectoryStopsTheStart(t *testing.T) {
 		filepath.Join(t.TempDir(), "missing"),
 		notADirectory,
 	} {
-		const want = "SWW_STATE_DIR cannot be written: "
+		const want = "SWW_WAF_STATE_DIR cannot be written: "
 
 		_, err := state.Load(newParams(dir))
 		if err == nil || !strings.HasPrefix(err.Error(), want) {