cli: make bool parsing case-sensitive

Completes the boolean fix: only literal "true"/"false" are accepted, with no
case folding, so "True"/"TRUE" are rejected. Drop strings.ToLower. Keep
TrimSpace so a config value with surrounding whitespace still parses; only the
case folding was invented leniency.

cli/options.go

@@ -38,8 +38,11 @@ var boolWords = map[string]bool{
 }
 
 // boolWord reports a value's truth and whether it is a recognised boolean word.
+// Whitespace is trimmed (aria2 strips config values too), but case is
+// significant: aria2 rejects "True"/"TRUE", so the match is against exactly
+// "true"/"false".
 func boolWord(s string) (val, ok bool) {
-	val, ok = boolWords[strings.ToLower(strings.TrimSpace(s))]
+	val, ok = boolWords[strings.TrimSpace(s)]
 	return val, ok
 }