Merge branch 'stage1-dont-kill-kthreads'

Merges pull request #15275:

    This addresses #15226 and fixes killing of processes before
    switching from the initrd to the real root.

    Right now, the pkill that is issued not only kills user space
    processes but also sends a SIGKILL to kernel threads as well.
    Usually these threads ignore signals, but some of these processes do
    handle signals, like for example the md module, which happened in
    #15226.

    It also adds a small check for the swraid installer test and a
    standalone test which checks on just that problem, so in the future
    this shouldn't happen again.

This has been acked by @edolstra on IRC.