aiccu: integrate with netifd

aiccu: integrate with netifd

This patch integrates AICCU with netifd. Care was taken not to restart
aiccu without a reason as it triggers alert on SixXS infrastructure.
Example usage:

config interface 'wan6'
        option 'proto'    'aiccu'
        option 'username' 'HANDLE-SIXXS/TID'
        option 'password' 'Password'
        option 'ip6prefix' '2001:db8:aabb::/48' #Delegated subnet
        option 'ip6addr' '2001:db8:aaaa:aaa::2/64' #Optional
        option 'verbose' 'true'

Tested with current trunk on TL-WR703N.

Signed-off-by: Ondrej Caletka <ondrej@caletka.cz>