Fix stunicast timer restart
Fix stunicast timer restart

Instead of hard coded setting the resend timer to CLOCK_SECOND,
restart it, so that the time between ANY following transmissions will
be as passed to stunicast_send_stubborn.

arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki