When AICCU is not running, running 'aiccu stop' results in unnecessary
communications with SixXS servers.

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

This check tries to wait for NTP daemon to synchronise clock to lower stratum
than 16 before running the aiccu binary. In case timeout (configurable
in new option ntpsynctimeout; default is 90 seconds) is reached, the
AICCU is run neverrtheless. This makes the new script compliant with
some corner case scenarios like not using Busybox NTPd or not using NTPd
at all, maintaining maximum out-of-the box functionality.

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

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

The option 'tunnel_id' from aiccu is called 'tunnelid' in the UCI config file.

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>