feat: add support for socket activation

[ruihe774]

This PR adds special handing for addresses in the form of /dev/fd/*. These addresses are treated as already listened. net.FileListener is used in this scenario instead of net.Listen. In this way, systemd socket activation can be supported.

For example, with v2ray.service installed, we can write a v2ray.socket:

[Unit]
Description=V2Ray Socket

[Socket]
ListenStream=127.0.0.1:1080
ListenStream=127.0.0.1:1081

[Install]
WantedBy=sockets.target

And set the listen addresses of the inbound to the activated sockets (starting from fd 3) in config.json:

// ...
{
    "proto": "http",
    "listen": "/dev/fd/3"  // this will be 127.0.0.1:1080
},
{
    "proto": "http",
    "listen": "/dev/fd/4"  // this will be 127.0.0.1:1081
}
// ...

v2ray.service will be automatically started by systemd once connections are made to 127.0.0.1:1080 and 127.0.0.1:1081.

Benefits of using socket activation:

• Support lazy startup of v2ray when needed
• Better performance when used in rootless container

XXX:

• Current implementation treats such addresses as UNIX domain socket despite of its real proto. The inbound proto must support UNIX ds to listen on such addresses.
• UDP (ListenDatagram=) is not supported yet.

[xiaokangwang]

I think there are some minor coding style issue with your current implementation of the feature. However, I believe this change is ready to be merged once these issue are addressed.

[xiaokangwang]

I have merged it! Thanks for your contribution.