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Otherwise, if we already have a packet in progress, finish it first, before
check "peer closed" status.

As Zephyr currently doesn't handle MTU itself (ZEP-1998), limit amount
of data we send on our side.

Also, if we get unsuccessful result from net_nbuf_append(), calculate
how much data it has added still. This works around ZEP-1984.

Instead of complicating recv() implementation.

For stream sockets, next exected operation is listen().

In this case, we can mark socket as closed directly.

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.

It will be reused e.g. for accept() implementation.

Short read approach is taken - at most, the remaining data in the current
fragment will be returned.

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.

So far, socket creation and closure is implemented.