Network: Implement user-mode networking

[jounathaen]

Currently, we support TAP networking, which requires root access. A user-mode networking would be desireable.

Hypervisors like QEMU use slirp for this, which is not trivial. But for a Unikernel, we always know which ports the application uses, so I guess we can get away with something simpler.

[fogti]

One should perhaps also consider passt...