cli: fix default flags when their command equivalents are disabled (#21469)

* always replace `\r\n` with `\n`

* fix

* test

* cleanup

* add doc comment

vlib/cli/testdata/default_command_no_flag_err.vv

@@ -0,0 +1,11 @@
+import cli { Command, CommandFlag }
+
+fn main() {
+	mut cmd := Command{
+		name: 'foo'
+		defaults: struct {
+			man: CommandFlag{true, false}
+		}
+	}
+	cmd.parse(['foo', '-man'])
+}