Make IPv6 NA multicast-to-unicast conversion configurable

Make IPv6 NA multicast-to-unicast conversion configurable

This can be used with Proxy ARP to allow multicast NAs to be forwarded
to associated STAs using link layer unicast delivery. This used to be
hardcoded to be enabled, but it is now disabled by default and can be
enabled with na_mcast_to_ucast=1. This functionality may not be desired
in all networks and most cases work without it, so the new
default-to-disabled is more appropriate.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>