MEDIUM: connection: Upstream SOCKS4 proxy support

MEDIUM: connection: Upstream SOCKS4 proxy support

Have "socks4" and "check-via-socks4" server keyword added.
Implement handshake with SOCKS4 proxy server for tcp stream connection.
See issue #82.

I have the "SOCKS: A protocol for TCP proxy across firewalls" doc found
at "https://www.openssh.com/txt/socks4.protocol". Please reference to it.

[wt: for now connecting to the SOCKS4 proxy over unix sockets is not
 supported, and mixing IPv4/IPv6 is discouraged; indeed, the control
 layer is unique for a connection and will be used both for connecting
 and for target address manipulation. As such it may for example report
 incorrect destination addresses in logs if the proxy is reached over
 IPv6]