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/tagdeny.go

@@ -1,6 +1,6 @@
 package waf
 
-// The heuristic tag_deny. It reads SWW_TAG_DENY.
+// The heuristic tag_deny. It reads SWW_WAF_TAG_DENY.
 
 import (
 	"context"
@@ -10,7 +10,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// tagDeny refuses a request that carries a tag in SWW_TAG_DENY. Its figure is
+// tagDeny refuses a request that carries a tag in SWW_WAF_TAG_DENY. Its figure is
 // the first such tag, in sorted order.
 type tagDeny struct{}