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/discriminator/countries_test.go

@@ -245,11 +245,11 @@ func TestExclusiveListRefusesAPrivateAddressUnlessAllowed(t *testing.T) {
 		action    string
 	}{
 		{
-			"not in SWW_ALLOW_NETS", "",
+			"not in SWW_WAF_ALLOW_NETS", "",
 			http.StatusForbidden, requestlog.ActionCountryDenied,
 		},
 		{
-			"in SWW_ALLOW_NETS", "10.0.0.7,fd00::/8",
+			"in SWW_WAF_ALLOW_NETS", "10.0.0.7,fd00::/8",
 			http.StatusOK, requestlog.ActionForward,
 		},
 	} {
@@ -279,7 +279,7 @@ func TestExclusiveListRefusesAPrivateAddressUnlessAllowed(t *testing.T) {
 // startGeoJS starts a stand-in for GeoJS, which places fromDE and fromKP,
 // each in an AS of its own, and no other address. It returns its URL, and
 // what returns the addresses it has been asked about. A test that needs
-// the stand-in to be asked or to answer sets SWW_LOOKUP_TIMEOUT to an
+// the stand-in to be asked or to answer sets SWW_WAF_LOOKUP_TIMEOUT to an
 // hour, whether or not a request waits for the answer: on the default, a
 // hold-up of the test process can abandon the request to the stand-in, and
 // leave the client unknown.