Headers seem to be a bit different in Apple land and you need to include
stddef.h explicitly to the NULL declaration.
This also makes the code a bit more correct, as stddef.h is the header
in the C standard that defines NULL
(https://en.cppreference.com/w/cpp/header/cstddef).
We need several FIFO queues (a queue of packets, a queue of messages,
etc.).
Some of them are implemented using cbuf, a generic circular buffer. But
for recording, we need to store the packets in an unbounded queue until
they are written, so the queue was implemented manually.
Create a generic implementation (using macros) to avoid reimplementing
it every time.