use /etc/os-release

use /etc/os-release

This should exist everywhere, its content look like shell
assignments but are limited enough to not require a shell to
parse (see os-release(5) or its parts pasted as a comment to
the parse function).
Further motivation for this is the fact that in suse the
/etc/SuSE-release file is declared deprecated in favor of
/etc/os-release.
Note that we have to read the file in a protected_call to
avoid symlink issues.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>