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/rulefiles.go

@@ -1,7 +1,7 @@
 package discriminator
 
 // The heuristic rule_files. It reads the rules of the rule files, which
-// SWW_RULES_DIR and SWW_RULES_ENABLED name.
+// SWW_WAF_RULES_DIR and SWW_WAF_RULES_ENABLED name.
 
 import (
 	"context"