systemd: also read /etc/sysconfig/lldpd

While systemd's author says that we should get rid of those
pseudo-configuration files, keeping compatibility with previous ways
to configure daemons seem a valid reason to keep them. `/etc/default`
is Debian specific. We also read the equivalent one for RedHat to
ensure that everything will also continue to work here.

 http://0pointer.de/blog/projects/on-etc-sysinit.html