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/crowdsec_test.go

@@ -18,8 +18,8 @@ import (
 // test puts in the copy of its decision list, at decisionsURL, that it
 // needs, as reputation.json would at start.
 const (
-	crowdSecURL  = "SWW_CROWDSEC_LAPI_URL"
-	crowdSecKey  = "SWW_CROWDSEC_LAPI_KEY"
+	crowdSecURL  = "SWW_WAF_CROWDSEC_LAPI_URL"
+	crowdSecKey  = "SWW_WAF_CROWDSEC_LAPI_KEY"
 	lapi         = "http://crowdsec.example:8080"
 	decisionsURL = lapi + "/v1/decisions"
 	bouncerKey   = "crowdsec-key-0123456789abcdef"