fix: no more Restricted Shell

[WaterLemons2k]

What does this PR do?

No longer running commands with Restricted Shell, as some Shells don't support it.

Determining whether a Restricted Shell is supported may add unnecessary complexity.

Fixes #771.

Motivation

#771

Additional Notes

The following code can be used to fallback when a Restricted Shell is not supported:

// run cmd
out, err := execCmd.CombinedOutput()
str := string(out)
if err != nil && strings.Contains(str, "bash: illegal option -r") {
	// 如果不支持受限 Shell 则使用 `bash -c` 运行命令
	// https://github.com/jeessy2/ddns-go/issues/771
	execCmd = exec.Command("bash", "-c", cmd)
	out, err = execCmd.CombinedOutput()

	if err != nil {
		log.Printf("获取%s结果失败! 未能成功执行命令：%s，错误：%q，退出状态码：%s\n", addrType, execCmd.String(), out, err)
		return ""
	}

} else {
	log.Printf("获取%s结果失败! 未能成功执行命令：%s，错误：%q，退出状态码：%s\n", addrType, execCmd.String(), out, err)
	return ""
}

[jeessy2]

考虑怎么改，要不要Restricted?

[WaterLemons2k]

考虑怎么改，要不要Restricted?

要么彻底抛弃，要么只在不支持 Restricted 的 Shell 中回退。

[jeessy2]

彻底抛弃吧。