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/waf/countries.go

@@ -1,6 +1,6 @@
 package waf
 
-// The heuristic countries. It reads SWW_EXCLUSIVELY_ALLOWED_COUNTRIES.
+// The heuristic countries. It reads SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES.
 
 import (
 	"context"
@@ -10,7 +10,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// countries refuses a request when SWW_EXCLUSIVELY_ALLOWED_COUNTRIES is set and
+// countries refuses a request when SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES is set and
 // does not hold the client's country as it was looked up. A client without a
 // country, or whose country cannot be found, is refused. Its figures are the
 // country and the setting that refused it.
@@ -33,7 +33,7 @@ func (countries) check(
 	return &verdict{
 		action: requestlog.ActionCountryDenied,
 		figures: map[string]any{
-			"country": country, "setting": "SWW_EXCLUSIVELY_ALLOWED_COUNTRIES",
+			"country": country, "setting": "SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES",
 		},
 	}
 }