Read every setting through viper and rename the settings (closes #211)

Every setting of waf 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: waf'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/metrics/busiest.go

@@ -43,7 +43,7 @@ func newCountries(topN int) *busiest {
 	countries := newBusiest(topN, "country", "the client's country")
 	countries.refused = counterVec(
 		"sww_country_list_refusals_total",
-		"Requests SWW_EXCLUSIVELY_ALLOWED_COUNTRIES refused, by the client's country.",
+		"Requests SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES refused, by the client's country.",
 		[]string{"country"},
 	)