Clear Linux containers running as root may have no /etc/passwd. But
they'll have /etc/machine-id because systemd creates that. Also test
/proc/version (a Linux-specific file) because that isn't writeable even
by root.
Take the opportunity to check with access() instead of assuming root and
only root can write to the file.
Change-Id: Ibdc95e9af7bd456a94ecfffd1603e8359604752b
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
7cd2d2b751