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/heuristics.go

@@ -37,7 +37,7 @@ const (
 )
 
 // The lists of heuristics, each in the order its heuristics run. A client in
-// SWW_ALLOW_NETS skips them all.
+// SWW_WAF_ALLOW_NETS skips them all.
 
 // beforeLookup are the heuristics that run before the client's AS number and
 // country are looked up, so that a client they refuse is not looked up.