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/coreruleset/drupal_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 )
 
-// drupalThreshold is SWW_WAF_ANOMALY_THRESHOLD's default.
+// drupalThreshold is SWW_WAF_CORE_RULE_SET_ANOMALY_THRESHOLD's default.
 const drupalThreshold = 5
 
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