lldpd: Make sure that the pidfile is created after forking for privilege separation

Currently when privilege is enabled, the pidfile contains the PID for the pre-fork PID. This causes any init system that relies on pidfiles to always fail to stop the daemon. This moves creation into the priv_init function, using an extra parameter to control whether it gets created or not.

This also moves starting of lldpcli to after daemonization on systems where daemonization is enabled.