Connect to gvproxy socket

[nirs]

Connecting to the socket set the remote address so we can use send()/recv() or write()/read() instead of sendto()/recvfrom().

Advantages:

• Simpler code, no need to keep the remote address.
• The server will want to connect to the client address to ensure it receives frames only from the connected client. Such server will want to remove the unix socket once the client connected[2], which doe snot work with current code.
• Once the socket is connected, the same backend can be used to handle passed file descriptor[1].

Testing:

• Tested with vmnet-helper
• Not tested yet with gvproxy

[1] containers/krunkit#24
[2] https://github.com/nirs/vmnet-helper/blob/5c6a595ba3e76314e1d0bef2b0160388439d69ec/helper.c#L475