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/refusallimit.go

@@ -1,7 +1,7 @@
 package discriminator
 
-// The heuristic refusal_limit. It reads SWW_ERROR_BURST_THRESHOLD and
-// SWW_BAN_RESPONSE.
+// The heuristic refusal_limit. It reads SWW_WAF_ERROR_BURST_THRESHOLD and
+// SWW_WAF_BAN_RESPONSE.
 
 import (
 	"context"
@@ -14,8 +14,8 @@ import (
 // refusalLimit counts the request once it has been answered, if discriminator
 // refused it after a rule file match, a trap path or a Core Rule Set match, and
 // in observe mode if enforce mode would have: more than
-// SWW_ERROR_BURST_THRESHOLD such refusals of the client within a minute break
-// a limit, which bans the client. A client in SWW_ALLOW_NETS, which the
+// SWW_WAF_ERROR_BURST_THRESHOLD such refusals of the client within a minute break
+// a limit, which bans the client. A client in SWW_WAF_ALLOW_NETS, which the
 // heuristics skip, is not counted, and nothing is while the threshold is off.
 // Its figures are the window, the count and the limit.
 type refusalLimit struct{}