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/denynets.go

@@ -1,6 +1,6 @@
 package discriminator
 
-// The heuristic deny_nets. It reads SWW_DENY_NETS.
+// The heuristic deny_nets. It reads SWW_WAF_DENY_NETS.
 
 import (
 	"context"
@@ -9,7 +9,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// denyNets refuses a client in SWW_DENY_NETS. Its figure is the netblock of
+// denyNets refuses a client in SWW_WAF_DENY_NETS. Its figure is the netblock of
 // the setting the client is in.
 type denyNets struct{}