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

Every setting of waf 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: waf'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_.

Model: deepseek-v4-pro (first version); opus-5-5 (rework)

internal/rules/rules.go

@@ -1,5 +1,5 @@
 // Package rules reads the rule files: the plain text files in
-// SWW_RULES_DIR, one rule to a line, that each request is checked
+// SWW_WAF_RULES_DIR, one rule to a line, that each request is checked
 // against, as the "Rule files" section of SPEC.md describes. They are read
 // at start, and again once the directory has had no change for a short
 // time after one is edited, added or removed.
@@ -40,7 +40,7 @@ const (
 // extension ends the name of every rule file.
 const extension = ".rules"
 
-// quietTime is how long SWW_RULES_DIR must go without a change before
+// quietTime is how long SWW_WAF_RULES_DIR must go without a change before
 // the rule files are read again, so that a file still being written, such
 // as one saved in place, appended to or copied in with scp, is read only
 // once whole.
@@ -103,9 +103,9 @@ type Rule struct {
 
 // Params are what Load needs.
 type Params struct {
-	// Dir is the directory of the rule files (SWW_RULES_DIR).
+	// Dir is the directory of the rule files (SWW_WAF_RULES_DIR).
 	Dir string
-	// Enabled is SWW_RULES_ENABLED: while it is false, no file is read
+	// Enabled is SWW_WAF_RULES_ENABLED: while it is false, no file is read
 	// and no rule loaded.
 	Enabled bool
 	// ProcessLog receives how many rules were read, and the error in a
@@ -274,7 +274,10 @@ func (f *Files) logRead(count int) {
 func read(dir string) ([]Rule, string, error) {
 	entries, err := os.ReadDir(dir)
 	if err != nil {
-		return nil, dir, fmt.Errorf("SWW_RULES_DIR cannot be read: %w", err)
+		return nil, dir, fmt.Errorf(
+			"SWW_WAF_RULES_DIR cannot be read: %w",
+			err,
+		)
 	}
 
 	var rules []Rule
@@ -307,7 +310,7 @@ func readFile(
 	rules []Rule,
 	places map[string]string,
 ) ([]Rule, error) {
-	//nolint:gosec // a rule file, in SWW_RULES_DIR
+	//nolint:gosec // a rule file, in SWW_WAF_RULES_DIR
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return nil, err