networking: add and implement net_addr_ll_set() API

networking: add and implement net_addr_ll_set() API

When running in TAP mode we may need to set the LL address of the
interface, if requested by the user.

This operation was overlooked when implementing the networking API and
it still relies on iproute/net-tools being installed.

Basically this means that when compiling OpenVPN on a system without
iproute2/net-tools and the user uses the "lladdr" config directive,
OpenVPN will fail to se the LL address of the interface.

With this patch a new API is introduced and it is implemented for both
SITNL and iproute2 backends.

Reported-by: Jan Hugo Prins <jprins@betterbe.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210903161113.30498-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22792.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>