Under ubuntu core/core-desktop, /etc/default/locale is not modifiable

I have a patch that fixes this. We are already using it in ubuntu core desktop. I'm preparing to upload it to the GIT repo.

We should really address the root of this problem instead of continuing to patch src:systemd.

We will definitely need to fix this in Jammy, but I am not sure it makes sense to do so in Mantic given the state of x11 keymap support (see bug 2030788).

In ubuntu core desktop, we need to be able to change these two files to allow to set the GDM keyboard and language.

> When working with ubuntu core or ubuntu core desktop, neither /etc/default/locale
> nor /etc/default/keyboard are modificable

They should be. This needs to be fixed in Ubuntu Core, not in systemd.

The point is that the way of fixing them is to make links to /etc/writable. But the systemd tools modify them by creating a new, temporary file first in the place, and then overwriting the old one with the new. So the patch does the same that was already done for other files: detect if the file is a soft link, and in that case, follow it up to the destination.

So it requires a fix both in Ubuntu Core and systemd.