Support for listening on unix sockets

[freman]

Fairly simply.

I'd like to spin up code-server on a unix socket which would allow me to create sockets fairly easily without ever risking a collision with another running instance.

You might ask how this is useful, well, I'm working on a custom sshcode that is cross platform and uses golang.org/x/crypt/ssh to do it's work (so even works on windows, with putty agent!).

I'm thinking it wouldn't take much more work that I've already put in to actually embed that in electron or lorca. Giving me a 'native' cross platform experience where I can work from my server.

(I of course would be releasing my code, atm it's just a mashed together mess but I'm using it now and it's working well)

[freman]

As promised, first pass WIP - https://github.com/freman/sshcode

Still very much sledge hammered together, but is capable of now taking sshkeys or using agents, in theory :D
Also uses lorca
Also uses sockets
Also cleans up after itself
Also treats flags similar to the way openssh would

todo: host key verification, cleaning, more structure, fix the map race condition on sessions, etc.