Conn proxy support of UNIX Sockets

[xSmurf]

UNIX Sockets are currently broken in the connection proxy.

[david415]

so far i only managed to write a proxy failure test:
https://github.com/david415/oz/tree/37.proxy_UNIX_sockets.0
https://github.com/david415/oz/blob/37.proxy_UNIX_sockets.0/network/proxy_test.go

[david415]

another work-in-progress branch here:
https://github.com/david415/oz/tree/37.proxy_UNIX_sockets.1

i removed the assertions that the specified UNIX domain sockets for proxying are abstract sockets; i do in fact want to proxy unix sockets that are present on the filesystem. unfortunately this didn't work:

May 25 16:11:08 subgraph oz-daemon[7506]: 2016/05/25 16:11:08 Starting socket client forwarding: unix:///var/run/tor/control.
May 25 16:11:08 subgraph oz-daemon[7506]: 2016/05/25 16:11:08 Unable to create connection proxy: {Nettype:client Proto:unix Port:0 Destination:/var/r
un/tor/control}, listen unix /var/run/tor/control: bind: address already in use

[david415]

after chatting with Smurf and reading this blog post:
https://integratedcode.us/2016/04/08/user-namespaces-sharing-the-docker-unix-socket/

i'm now convinced that what we must do is:

1. oz-init creates an abstract UNIX domain socket in the sandbox and proxies this to the filesystem UNIX domain socket on the host (e.g. /var/run/tor/control);
2. another proxy daemon inside the sandbox creates the filesystem UNIX domain socket (e.g. /var/run/tor/control) and proxies connections to oz-init's abstract UNIX domain socket which then proxies to the host's socket.

[david415]

here's a pull-request for a "good enough" solution; proxy TCP from inside Oz to a UNIX socket on the host --> #65
please review this. maybe don't just merge it... i removed some of the proto prefix checking and i guess maybe we might want to keep that logic if we want to support TCP/IPv6? seems unlikely but just maybe i broke something... though it works for me.

[dma]

This works and has been deployed for some time, closed.