Internal structure of k_fifo changed between 1.7 and 1.8, so we need
to abstract it away. This adds more functions than currently used, for
future work.
The foundation of recv() support is per-socket queue of incoming packets,
implemented using Zephyr FIFO object. This patch implements just recv()
for UDP, because TCP recv() requires much more fine-grained control of
network fragments and handling other issues, like EOF condition, etc.
