shellcheck: minor fixes to support all dsitros

checked with::

  sudo ./utils/lxc.sh cmd -- make test.sh

on ubu1804, ubu1910, fedora31 and archlinux.  All checks OK except:

ubu1604: uses shellcheck v0.3.7 (from 04/2015) which is no longer supported!

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>