Provide a straightforward way to reset networking after initramfs

[Johan Ehnberg]

Since initramfs networking is rather limited, a simple flag to reset networking afterwards (or not to generate netplan files) would be valuable. The intended effect is to allow fully featured networking tools to set up the network afterwards.

Use cases include the following, in comination with networking-enabled initramfs:
* Multiple IPs on a single interface
* Adding search domains
* Unlocking root over SSH on a different network than will be used afterwards
* Using DHCP for initramfs and static later

There are numerous workarounds floating around, as well as questions on this behaviour. Netplan has made this even more difficult to get around, since it does not provide an equivalent to 'service networking restart'. As of Ubuntu 20.04, I have not even found any workaround that actually works.

Related:
dropbear-initramfs IFDOWN not working LP #1813394
Alternative approach with netplan in initramfs LP #1880717
What is creating /run/netplan/eth0.yaml? askubuntu #1228433

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.