sd-lldp: several improvements (#37845)

This makes
- sd-lldp-tx not send machine ID as chassis ID, but use application
specific machine ID,
- sd-lldp-tx emit vlan ID if it is running on a vlan interface,
- Describe() DBus method also reply LLDP configurations,
- io.systemd.Network.GetLLDPNeighbors varlink method provides vlan ID,
if received.

Closes #37613.