doc: add documentation about reading /etc/lldpd.conf and /etc/lldpd.d/*
build: check if libevent as provided by pkg-config is working

If a user installs both libevent 2.0.x and libevent 1.4.x, pkg-config
may report libevent 2.0.x where it really is 1.4.x. The system is
broken in this case but we work-around this by using the shipped
libevent.