zfs.NilBool: document its purpose and move it to its own package 'nodefault'

util/nodefault/nodefault_bool.go

@@ -0,0 +1,19 @@
+package nodefault
+
+import "fmt"
+
+type Bool struct{ B bool }
+
+func (n *Bool) ValidateNoDefault() error {
+	if n == nil {
+		return fmt.Errorf("must explicitly set `true` or `false`")
+	}
+	return nil
+}
+
+func (n *Bool) String() string {
+	if n == nil {
+		return "unset"
+	}
+	return fmt.Sprintf("%v", n.B)
+}