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/ratelimit/ratelimit_test.go

@@ -13,7 +13,7 @@ import (
 const limit = 3
 
 // tableSize is the most clients the tests' tables hold, the default of
-// SWW_MAX_TRACKED_CLIENTS.
+// SWW_WAF_MAX_TRACKED_CLIENTS.
 const tableSize = 20000
 
 // whole is the percentage of each limit a client gets when nothing lowers