Allow configuring tx-interval in milliseconds

Allow configuring tx-interval in milliseconds

This is a prerequisite for TTDP support.

Usage: add "ms" suffix to the delay value to force interpretation as a
milliseconds value:

 lldpcli configure lldp tx-interval 200ms

When computing a TTL (TTL = tx-interval * tx-hold), the result is rounded
up to the next second, as LLDP frames need a value in seconds.

A new tx-interval-ms item is added to the json output. The old tx-interval
is kept in seconds (rounded up) for compatibility.