Pass undocumented argument '--fast-open' when execute sip003 plugin

[qiuyuzhou]

shadowsocks-libev/src/plugin.c

Lines 114 to 121 in 870f9d9

	extern int fast_open;
	if (fast_open)
	cork_exec_add_param(exec, "--fast-open");
	#ifdef __ANDROID__
	extern int vpn;
	if (vpn)
	cork_exec_add_param(exec, "-V");
	#endif

I found this when I develop a new plugin. Then the plugin must could handle this undocumented flag. I think this is not a good approach. Many arguments parse libraries would reject unknown flag.

 2020-04-16 17:20:08 INFO: plugin "plugins/h2tun" enabled
 2020-04-16 17:20:08 INFO: using tcp fast open
 2020-04-16 17:20:08 INFO: initializing ciphers... chacha20-ietf-poly1305
 2020-04-16 17:20:08 INFO: listening at 127.0.0.1:1086
 2020-04-16 17:20:08 INFO: tcp port reuse enabled
Error: unknown flag: --fast-open
Usage:
  h2tun [flags]

Flags:
  -h, --help                 help for h2tun
      --local-host string     (default "127.0.0.1")
      --local-port uint16     (default 18086)
  -p, --path string          Handle tunnel at the path (default "/h2tunnel")
      --remote-host string    (default "127.0.0.1")
      --remote-port uint16    (default 18096)
  -s, --server               Run in server mode
  -v, --version              version for h2tun