lldpd: Add tx_hold

This patch adds a new parameter tx_hold and also
changes txTTL calculation to be a multiple of
tx_hold and tx_interval

Also adds cli support to configure tx-hold:
#lldpcli configure lldp tx-hold 1

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
man: tx-interval cannot be set per port
man: document tx-hold