lldpd: invoke lldpcli at start

To configure itself, lldpd now invokes lldpcli when
starting. Currently, it does not really check if everything works as
expected but in the future, lldpcli will ask lldpd to "start".

The configuration files are:
 - ${sysconfdir}/lldpd.conf
 - ${sysconfdir}/lldpd.d/*.conf
lldpd: pause at start, unpause through lldpcli

lldpd will be paused at start. Nothing will run. lldpcli job is to
unpause it once the configuration has been read.
lldpd: pause only if lldpcli has not been disabled
lldpd: discard output of lldpcli

We keep stderr since we may have logs here.