ifconfig is effectively deprecated for quite some time now. Let's
replace the remaining occurrences for packages by the
corresponding ip commands now.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Also ensure that the error message is actually printed to stderr and that
the rule generation is aborted if an interface cannot be resolved.
Ref: https://github.com/openwrt/luci/issues/3975
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
