RFE: slirp4netns support

[gasinvein]

Using userspace network stack like slirp4netns as an alternative to sharing network namespace could provide a more secure way to connect sandboxed apps to the internet:

• No abstract sockets leak (#330)
• No access to the "real" localhost and services listening on it (with --disable-host-loopback) (#340)
• Apps can't receive incoming connections unless there is explicit port forward (this may be a disadvantage, though)

[pwFoo]

slirp4netns or another network solution would be nice! Any plans to implement?