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/lookup_test.go

@@ -20,7 +20,7 @@ import (
 // and country.
 type asnAndCountry struct{ asn, asName, country string }
 
-// fileSource is the SWW_LOOKUP_SOURCE that looks clients up in the
+// fileSource is the SWW_WAF_LOOKUP_SOURCE that looks clients up in the
 // lookup database.
 const fileSource = "file"