Support for modifying PortID TLV

This patch adds the ability to change the behavior of what
lldpd sends in the PortID TLV.

By default, lldpd sets the MAC address in the PortID and the ifname
in the PortDescr field.  This patch allows the user to
configure lldpd via the CLI (lldpcli or /etc/lldpd.conf)
to send either the interface name
(ifname) or the MAC address in the PortID TLV.

Signed-off-by: Sam Tannous <stannous@cumulusnetworks.com>
lldpcli: remove unused variable
lldpcli: fix a format warning
NEWS: mention ability to configure port ID
lldpcli: document `configure lldp portidsubtype` command