Rework the bonding.sh protocol handler to accept slave interface names
encoded in uci list notation. Also replace ifconfig up/down with ip
link calls while we're at it.

Fixes: #11455
Fixes: https://github.com/openwrt/luci/issues/4473
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

Fix proto_notify_error usage. In this function only a definition shall
be returned, which will later appear in the ubus status output of the
interface and not a whole string.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

Signed-off-by: Helge Mader <ma@dev.tdt.de>

Signed-off-by: Helge Mader <ma@dev.tdt.de>