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. 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 tests pass set into a viper instead of a lookup function.

Model: deepseek-v4-pro

docs/settings.md

@@ -354,9 +354,8 @@ asked in the background (see "Answering in time" under "`servewaf`" in
 ## `rproxy`
 
 `smallwebwaf rproxy` reads the settings below and no others. They are read once
-at start, and an invalid value stops the start. None can be given as a file,
-since `rproxy` never touches disk. "`rproxy`" in [`SPEC.md`](../SPEC.md) says
-what they do.
+at start, and an invalid value stops the start. Any can be given as a file, as a
+`SWWAF_` setting can. "`rproxy`" in [`SPEC.md`](../SPEC.md) says what they do.
 
 A time here is a whole number of milliseconds above zero, such as `2000`.