In some cases booting firware requires multiple IPv6 addresses leased
from DHCP. Dnsmasq offers the same address to different IAIDs requests.
The first gets it successfully, but remaining requests would be denied
with address in use error. Change behaviour and supply alternative
address from the same range, just like Rapid Commit DHCP option allows.
Reuse the same code.
Resolves: rhbz#2002871
