Move most of gvproxy code to a reusable go package with an API

gvproxy code:

1. register/parses command line arguments
2. creates a types.Configuration instance from these args
3. creates some listeners/http muxers from these args + associated go routines

All of this is done in 2 big functions. The crc daemon does 2) and 3) but with slight variations, and without sharing any code from gvproxy. Following https://github.com/containers/gvisor-tap-vsock/pull/280 , it might be useful to podman as well to have an easy way to reimplement their own gvproxy + 9p binary.

What I aim to do is to move 2) and 3) to their own go package (maybe pkg/usernet) which can be used by both gvproxy and the crc daemon. Adding an API for 2) is mostly done, 3) is still being investigated.