Add enable_disable() helper

In this patch "enabled" and "disabled" is used exclusively, but "enable" and
"disable" forms are need for the following patch.

src/boot/bootctl.c

@@ -1094,7 +1094,7 @@ static int verb_status(int argc, char *argv[], void *userdata) {
                 if (r < 0)
                         log_warning_errno(r, "Failed to query secure boot status: %m");
                 else
-                        printf("  Secure Boot: %s\n", r ? "enabled" : "disabled");
+                        printf("  Secure Boot: %sd\n", enable_disable(r));
 
                 r = is_efi_secure_boot_setup_mode();
                 if (r < 0)