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

For the first case, we call the function associated to the signal
ourselves after the signal is configured. This function is idempotent.

For the second case, we setup the SIG_CHLD signal even when privilege
separation is disabled.

Fix #250