libsystemd-network: introduce lldp lib and tests

This patch set introduces the Link Layer Discovery Protocol (LLDP)
specified in this standard allows stations attached to an IEEE
802 ® LAN to advertise, to other stations attached to the same IEEE 802
LAN, the major capabilities provided by the system incorporating that
station, the management address or addresses of the entity or
entities that provide management of those capabilities, and the
identification of the station’s point of attachment to the IEEE 802
LAN required by those management entity or entities.
libsystemd-network: Add lldp network functions

This patch add the lldp raw socket init and receive functions
libsystemd-network: add lldp port function

This patch adds the lldp port and functions
libsystemd-network: Add internal lldp functions and MIB

This patch adds Remote MIB DB and tlv read functions.
libsystemd-network: introduce lldp callbacks sd-lldp

Export lldp functions to be called from networkd
networkd: integrate lldp with networkd
networkd man: add lldp description