Configure /etc/resolv.conf and persist hostname
live-boot generates /etc/hostname, /etc/hosts, and
/etc/resolv.conf in do_netsetup from 9990-networking.sh.
kdump-tools needs similar code for generating a self-contained initrd
for dumping a crashed kernel via network.
Ubuntu carries a patch for initramfs-tools that adds
netinfo_to_resolv_conf to configure /etc/resolv.conf which is more
complex than the code from live-boot.
To prevent code duplication, implement netinfo_to_resolv_conf and
persist_hostname as helper functions and let configure_networking
call these functions.
Let netinfo_to_resolv_conf support multiple /run/net-<device> style
files to allow Ubuntu to reuse this code.
The second commit adds unit tests for netinfo_to_resolv_conf.
Edited by Benjamin Drung