net: shell: Allow dynamic interface name expansion

Allow multiple commands use the same dynamic shell command
completion when expecting network interface index.

For example "net iface" and "net stats" are such commands.

The network interface expansion cannot be used in "net ipv6 add",
"net ipv4 add" and "net route" commands as they require more
data after the network interface index argument.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>