lldpd: correctly parse /etc/os-release

The previous parsing was just bogus and eated latest characters.
test: add an helper to generate many neighbors

Using scapy, we generate a pcap with as many neighbors we need.
lib: fix sync_recv to handle segmentation

While it should not happen with Unix sockets, a read() can return only
partial data. Therefore, we continue to poll until we get the
appropriate length.

This fix was proposed by Chris Yang. Closes #31.