ipfw3: new feature 'ipfwsync'

ipfw3: new feature 'ipfwsync'

ipfwsync is a new feature to synchronize firewall states between machines
which is running ipfw3 firewall for high availability. ipfw3 can be configured
in centre or edge mode. the centre will automatically sync the states from
centre to the edge.

e.g.
ipfw3 sync edge 5000
ipfw3 sync start edge

ipfw3 sync centre 192.168.1.1:5000,192.168.1.2:5000
ipfw3 sync start centre

ipfw3 sync show config
ipfw3 sync show status
ipfw3 sync test centre 123