`examples/gnrc_networking_mac/udp.c` is a duplicate of the file in
`gnrc_networking`. However, while the latter moved along in code style
and static analysis, the first was left a bit at the wayside. This
change resyncs the two files.
The NIB's forwarding engine behaves more correctly than GNRC's old
forwarding engine in that link-local addresses require also an
interface to be provided to be meaningful. As with other commands this
is simplified for the special case that there only is one interface
that that interface is chosen.
