The rational is that for devices with dual addressing modes (or more) as
for example IEEE 802.15.4 you can get or set these addressing modes for
the source address of a packet. The length of the destination address is clear
since it usually arrives through the `ng_netif_hdr_t`.
In case of IEEE 802.15.4 this is needed to determine the compression
state of source IPv6 addresses in 6LoWPAN IPHC.
