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

Every setting of discriminator 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. Discriminator'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_.
Every place a name appeared changed: the code, the viper keys, tests,
documents, examples, the README's settings table and the image files.

Model: deepseek-v4-pro

internal/rproxy/bodies.go

@@ -10,16 +10,16 @@ import (
 )
 
 // errResponseTooLarge ends an app's response body that is longer than
-// SWRP_MAX_RESPONSE_SIZE.
+// SWW_RPROXY_MAX_RESPONSE_SIZE.
 var errResponseTooLarge = errors.New(
-	"the response body is over SWRP_MAX_RESPONSE_SIZE")
+	"the response body is over SWW_RPROXY_MAX_RESPONSE_SIZE")
 
 // requestBody is the visitor's request body. rproxy first reads up to the first
 // 64 KiB of it into held, and then, on its way to the app, the transport reads
 // it on a goroutine of its own.
 type requestBody struct {
 	// body is the visitor's body, ending in an *http.MaxBytesError past
-	// SWRP_MAX_REQUEST_SIZE.
+	// SWW_RPROXY_MAX_REQUEST_SIZE.
 	body io.ReadCloser
 	rq   *request
 	// held is the part read before the request goes to the app, which Read gives
@@ -68,7 +68,7 @@ func (b *requestBody) Close() error {
 // responseBody is the app's response body on its way to the visitor.
 type responseBody struct {
 	// body is the app's body, ending in an *http.MaxBytesError past
-	// SWRP_MAX_RESPONSE_SIZE.
+	// SWW_RPROXY_MAX_RESPONSE_SIZE.
 	body io.ReadCloser
 	rq   *request
 }