networkd: Introduce Link Layer Discovery Protocol (LLDP)

This patch introduces LLDP support to networkd. it implements the
receiver side of the protocol.

The Link Layer Discovery Protocol (LLDP) is an industry-standard,
vendor-neutral method to allow networked devices to advertise
capabilities, identity, and other information onto a LAN. The Layer 2
protocol, detailed in IEEE 802.1AB-2005.LLDP allows network devices
that operate at the lower layers of a protocol stack (such as
Layer 2 bridges and switches) to learn some of the capabilities
and characteristics of LAN devices available to higher
layer protocols.
networkd: integrate LLDP

This patch integrates LLDP with networkd.

Example conf:
file : lldp.network

[Match]
Name=em1

[Network]
LLDP=yes
LLDP: Add support for networkctl
networkctl: lldp - respect arg_legend
networkd: link - plug leak