priv: correctly handle lldpcli exit

priv: correctly handle lldpcli exit

When lldpcli exits, we must acknowledge its death with
`waitpid()`. There were two missing cases:

 - when lldpcli exits before setting the SIG_CHLD signal
 - when privilege separation was not configured

To fix both cases, we ask the OS to reap children for us. We know when
monitored process dies because we are unable to read from pipe (no
need for a signal).

To fix the first case, we just reap any existing dead child after
setting the signal.

Fix #250