Read every setting through viper (closes #211)

servewaf's and rproxy's settings are read through viper, bound to the same environment variable names as before, 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, as it did. The _FILE forms, their errors, and rproxy reading no file form are unchanged, and the settings tests pass set into a viper instead of a lookup function.

Model: deepseek-v4-pro

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 servewaf 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
+}