Fix for “ipv6: Neighbour table overflow.” messages in syslog

Read http://cosu.ro/blog/2011/09/26/ipv6-neighbour-table-overflow/.

For Debian, I advice to place the sysctl parameters in /etc/sysctl.d/ipv6-neighbour-table-thresholds.conf and then execute:

sysctl -p /etc/sysctl.d/ipv6-neighbour-table-thresholds.conf