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_.
Every place a name appeared changed: the code, the viper keys, tests,
documents, examples, the README's settings table and the image files.

Model: deepseek-v4-pro

internal/ratelimit/ratelimit.go

@@ -2,7 +2,7 @@
 // and bytes counted over a minute, an hour and a day, as the "Counting
 // method" section of SPEC.md describes, which tell when a request takes
 // the client over a rate limit or a byte limit, and each client's history
-// since it was first seen. At most SWW_MAX_TRACKED_CLIENTS clients are
+// since it was first seen. At most SWW_WAF_MAX_TRACKED_CLIENTS clients are
 // kept, in memory, and written to clients.json and read from it by the
 // state package.
 package ratelimit
@@ -156,7 +156,7 @@ type Request struct {
 }
 
 // New returns a Limiter for limits, with no client counted yet, whose
-// table holds at most maxClients clients (SWW_MAX_TRACKED_CLIENTS). Past
+// table holds at most maxClients clients (SWW_WAF_MAX_TRACKED_CLIENTS). Past
 // it, the least recently seen client is dropped, with its history, and
 // starts afresh if it comes back.
 func New(limits Limits, maxClients int) *Limiter {