Fixes for ASIO >= 1.13 + fixed assertion when parsing SOCKS v header

[bgolinvaux]

Recent versions of ASIO (I am using Boost 1.77) have removed the get_io_service() method. The get_executor() method can be used instead.

Also, in some cases, in Proxy::ParseV5Header, when copying parts of buffer to hdr_buffer, the remaining size was 0 and the C debug runtime complained because we took the address of the first element of the empty buffer (&hdr_buffer[0])

The first change is risky because it bumps the version requirement on ASIO, while the second change looks harmless.

[jgaa]

Looks good. I'll just have to test it with various versions of Linux. (Sorry for l o n g delay. I've been buried in work)