This allows selecting the propagation level of emitted LLDP packets
(specifically: the destination MAC address of the packets). This is useful
because it allows generating LLDP packets that optionally cross certain types
of bridges.
See 802.11ab-2009, Table 7-1 for details.
Let's add some minimalistic LLDP sender support. The idea is that this is
either on or off, and all fields determined automatically rather than
configured explicitly.