improve lldp state engine

[sspans]

The current model isn't very flexible and doesn't meet the lldp(-med) requirements. With the new child_send it now is possible to generate packets for individual interfaces, which in turn makes it possible to be a lot more flexible with other parts of the code.

• further generalize child_send
• investigate lldp(-med) requirements
• implement trigger based on received packet
• implement support for shutdown frame (ttl 0)