Add support for sysvinit

[ZhaoXiangXML]

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Use the commands below to provide key information from your environment:
You do NOT have to include this information if this is a FEATURE REQUEST

What version of frp are you using (./frpc -v or ./frps -v)?
0.29.0

What operating system and processor architecture are you using (go env)?
Linux localhost 4.4.23+ #1 SMP PREEMPT Wed Nov 28 00:44:03 CST 2018 armv7l armv7l armv7l GNU/Linux

It's an Android 4.4 phone with Linux Deploy

Configures you used:

Steps to reproduce the issue:

1. systemctl start frpc

Describe the results you received:
System has not been booted with systemd as init system (PID 1). Can't operate.

Describe the results you expected:
run frpc as service

Additional information you deem important (e.g. issue happens only occasionally):

Can you point out what caused this issue (optional)
According to meefik/linuxdeploy#846, it's nearly impossible to run systemd as init system on Android.

Is it possible to provide scripts for sysvinit?

Thanks

[ZhaoXiangXML]

总之就是在没有systemd的情况下如何在后台运行frpc？我的客户端是装在安卓手机上的，似乎没有办法用systemd。

[ZhaoXiangXML]

我发现我的路由器上自带了frpc，也配置成功了。那就不需要这个了