lldpcli: Add support to display lldp stats

This patch adds a new lldpcli command and required
server code to display per interface lldp stats

Sample output:

#lldpcli show statistics ports eth0
---------------------------------------------------
LLDP statistics:
---------------------------------------------------
Interface:    eth0
  tx_cnt:       2
  rx_cnt:       2
  rx_discarded_cnt: 0
  rx_unrecognized_cnt: 0
  ageout_cnt:
  insert_cnt:
  delete_cnt:   0
---------------------------------------------------

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
event: add missing <time.h> include
lldpcli: reuse cmd_restrict_ports() to handle port restriction in "show stats"
lldpcli: use human labels for statistics

Also fix display of "Ageout" and "Inserted".
lib: remove unneeded struct for stats in private.h
doc: update NEWS entry for "show statistics" command