IPaddr2: Allow to send IPv6 Neighbor Advertisements in background

"Starting" an IPv6 address with IPaddr2 involves sending Neighbor
Advertisement packets to inform neighboring machines about the new
IP+MAC translation. By default, 5x packets with 200 ms sleep after each
are sent which delays the start by 1000 ms. Allow the user to run this
operation in background, similarly as is possible with GARP for IPv4.