Fix shell coding and style issues in the ifupdown scripts

Suggested-by: bashate & shellcheck
See-also: https://www.shellcheck.net/wiki/SC2003
See-also: https://www.shellcheck.net/wiki/SC2006
See-also: https://www.shellcheck.net/wiki/SC2034
See-also: https://www.shellcheck.net/wiki/SC2086
See-also: https://www.shellcheck.net/wiki/SC2162
See-also: https://www.shellcheck.net/wiki/SC2166