Fix command line handling in tunslip6
Fix command line handling in tunslip6

Signed-off-by: Maxim Osipov <3be22ebcf69784a7f74fbf51168974077a63c594@gmail.com>
Signed-off-by: Mariano Alvira <1418c40237ee713b2752a18beb0b3335c688b68b@devl.org>

bluerover/6lbr,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,arurke/contiki,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,bluerover/6lbr