99base: Rework `/etc/initrd-release` to derive from real os-release

99base: Rework `/etc/initrd-release` to derive from real os-release

I'd like to rework CoreOS Ignition (which runs in the initramfs)
to include some values from the *real* `/etc/os-release` in
HTTP headers.

Looking at this, it turns out dracut eats almost all of the useful
information from it.  I don't think `dracut` should be the `ID`
here...dracut's not an OS itself, it's a way to *build* little
operating systems.  It'd be kind of like if Fedora's Koji
injected itself into `/etc/os-release`.

This code dates back a long time; not sure of all the rationale
behind it.

I changed it so that we keep extending the VERSION/PRETTY_NAME
with the dracut version, but otherwise "pass through" the
rest of the real `/etc/os-release` we were built from unchanged.