ssh-proxy: add ssh ProxyCommand tool that can connect to AF_UNIX + AF_VSOCK sockets

ssh-proxy: add ssh ProxyCommand tool that can connect to AF_UNIX + AF_VSOCK sockets

This adds a tiny binary that is hooked into SSH client config via
ProxyCommand and which simply connects to an AF_UNIX or AF_VSOCK socket
of choice.

The syntax is as simple as this:

     ssh unix/some/path     # (this connects to AF_UNIX socket /some/path)

or:

     ssh vsock/4711

I used "/" as separator of the protocol ID and the value since ":" is
already taken by SSH itself when doing sftp. And "@" is already taken
for separating the user name.