pass SUBREAPER to prctl patch and new desktop flavor

[crazy-max]

Expected to fail with current qemu (7.1.0): https://github.com/tonistiigi/binfmt/actions/runs/4938696451/jobs/8832456122?pr=134#step:6:49

#9 [4/4] RUN /tini -s -- uname -a
#0 0.055 [FATAL tini (1)] PR_SET_CHILD_SUBREAPER is unavailable on this platform. Are you using Linux >= 3.4?

@tonistiigi has bisected it to: qemu/qemu@220717a

220717a6f46a99031a5b1af964bbf4dec1310440 is the first bad commit
commit 220717a6f46a99031a5b1af964bbf4dec1310440
Author: Richard Henderson <richard.henderson@linaro.org>
Date:   Mon Dec 27 07:01:23 2021 -0800

    linux-user: Disable more prctl subcodes

Create a list of subcodes that we want to pass on, a list of
subcodes that should not be passed on because they would affect
the running qemu itself, and a list that probably could be
implemented but require extra work. Do not pass on unknown subcodes.

moving the SUBREAPER constants from the EINVAL case to the prctl() case seems to fix the issue but might not be safe as the commit message suggests (edited)

series: https://lore.kernel.org/all/20211015041053.2769193-1-richard.henderson@linaro.org/T/

[crazy-max]

@tonistiigi @djs55 suggested patch looks to work indeed: https://github.com/tonistiigi/binfmt/actions/runs/4950685510/jobs/8854647906#step:8:9

Linux 90a30464b502 5.15.0-1037-azure #44-Ubuntu SMP Thu Apr 20 13:19:31 UTC 2023 aarch64 Linux