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/config/viper.go

@@ -0,0 +1,16 @@
+package config
+
+import "github.com/spf13/viper"
+
+// NewViper returns the viper that reads the settings: each setting's
+// environment variable, bound to its own name. Every setting of waf
+// and rproxy is read through it, and none is read with os.Getenv or
+// os.LookupEnv outside it. An environment variable that is set but empty is
+// set, as one that holds a value is, which viper does not take for granted.
+func NewViper() *viper.Viper {
+	v := viper.New()
+	v.AutomaticEnv()
+	v.AllowEmptyEnv(true)
+
+	return v
+}