Read every setting through viper and rename the settings (closes #211)

Every setting of waf 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: waf'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/waf/ratelimits.go

@@ -1,7 +1,7 @@
 package waf
 
-// The heuristic rate_limits. It reads SWW_RATE_LIMIT_EXEMPT_NETS,
-// SWW_RATE_LIMIT_EXEMPT_PATHS and SWW_BAN_RESPONSE, and the rate limits and
+// The heuristic rate_limits. It reads SWW_WAF_RATE_LIMIT_EXEMPT_NETS,
+// SWW_WAF_RATE_LIMIT_EXEMPT_PATHS and SWW_WAF_BAN_RESPONSE, and the rate limits and
 // biased thresholds through the limiter and limitPercentages.
 
 import (
@@ -63,8 +63,8 @@ func (rq *request) overAWholeRateLimit(now time.Time) bool {
 }
 
 // countedByRateLimits reports whether the rate limits count the request: its
-// client is not in SWW_RATE_LIMIT_EXEMPT_NETS, and its path is not exempt
-// under SWW_RATE_LIMIT_EXEMPT_PATHS.
+// client is not in SWW_WAF_RATE_LIMIT_EXEMPT_NETS, and its path is not exempt
+// under SWW_WAF_RATE_LIMIT_EXEMPT_PATHS.
 func (rq *request) countedByRateLimits() bool {
 	cfg := rq.h.config