Make the Chrome sandbox work

On Debian it needs `sysctl -w kernel.unprivileged_userns_clone=1` but on FreeBSD we get `/proc/sys/kernel/unprivileged_userns_clone: No such file or directory`.

Workaround:
Launch Chromium/Electron based applications with `--no-sandbox --no-zygote --test-type --v=0 "$@"`. Which is not ideal because a) it disables the sandbox b) it shows a warning message in some application GUIs, confusing users c) many existing AppImages don't come like that

cc @trasz

Reference:
https://wiki.freebsd.org/LinuxApps

Start by comparing the Debian sysctl requirement with the FreeBSD failure described in the issue, and read the linked FreeBSD LinuxApps reference. Reproduce the Chromium/Electron launch behavior using the documented workaround; done means the sandbox works on FreeBSD without disabling it or producing the reported GUI warning.

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