contiki-ng: It is not safe to call process_post in an interrupt handler

I’m not sure if this is expected or not, but it is not safe to call process_post in an interrupt handler. This is because it may cause inconsistency in the event queue (in sys/process.c).

The consumer of the event queue is do_event function (run in normal context), and the producer is process_post function. Because operation on fevent and nevents are not atomic in do_event, in an interrupt handler process_post may manipulate the event queue while it’s in an inconsistent state. This may cause losing a posted event or dispatching an event that was already dispatched before.

I don’t think so many modules call process_post in an interrupt context, but at least the tsch module does (in tsch_disassociate, which may be called in tsch_slot_operation thread) (although that is relatively a rare case).

I think we should either

1. make the event queue thread-safe, probably by using ringbufindex.
2. or, add warning to the document of process_post, and fix existing calls to process_post in interrupt contexts. For the tsch case above, we can use process_poll instead.