socks5.tcl: handle the -async option for `socket`
README.textile: add the `-async` option and handle it in the sample
socks5.tcl: handle but ignore the -async option for socket

The socket is used synchronously after being created, so the -async
option is useless. However it remains legal and must still be handled by
getopt.
Merge branch laomaiweng/socks5/socket-async