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/lookup/file_internal_test.go

@@ -99,9 +99,9 @@ func TestFileThatCannotBeReadIsAnError(t *testing.T) {
 	writeFile(t, notDatabase, "not a lookup database\n")
 
 	for path, want := range map[string]string{
-		missing: "SWW_LOOKUP_DB_PATH cannot be read: open " + missing +
+		missing: "SWW_WAF_LOOKUP_DB_PATH cannot be read: open " + missing +
 			": no such file or directory",
-		notDatabase: "SWW_LOOKUP_DB_PATH " + notDatabase +
+		notDatabase: "SWW_WAF_LOOKUP_DB_PATH " + notDatabase +
 			" is not a .mmdb file: error opening database: invalid MaxMind DB file",
 	} {
 		_, err := OpenFile(FileParams{
@@ -216,7 +216,7 @@ func TestReplacementThatCannotBeReadLeavesTheFileInUseWithOneAlert(t *testing.T)
 			Reason: "the lookup database cannot be read, and the one read before stays in use",
 			Detail: map[string]any{
 				"file": path,
-				"error": "SWW_LOOKUP_DB_PATH " + path + " is not a .mmdb file: " +
+				"error": "SWW_WAF_LOOKUP_DB_PATH " + path + " is not a .mmdb file: " +
 					"error opening database: invalid MaxMind DB file",
 			},
 		})