net: shell: Prevent deadlock with net arp command

In case one of the networking shell backends is enabled, net arp command
could potentially trigger a deadlock, as it locks the ARP mutex before
TCP connection mutex, while TCP stack could do this in reverse order
(for instance when sending ACK or retransmission).

Mitigate this, by forcing a separate TX thread in such case, so that ARP
mutex is no longer accessed with TCP mutex locked.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>