client: add show interfaces command (#240)

client: add show interfaces command (#240)

There are several lldpcli configure commands that take port name(s)
as an optional parameter.

Currently, it is not possible to verify interface specific configuration
for lldpd without access to a neighbour switch/host or packet capture.

It would be useful to have a show interfaces command, similar to the
show neighbors command that allows a user/agent to query the local
TLV information.

This commit leverages the existing port display infrastructure to
enable display of local interfaces.  There are a few differences:

- Do not display the chassis RID, as the local index is always 0
- Show TTL as a seperate TLV based on tx_hold, tx_interval config,
  as internally the port TTL atom info is 0, and the chassis TTL
  atom info has been depreciated.