lib: posix: eventfd: general eventfd improvements

The eventfd implementation suffers from various shortcomings
and it is not thread safe.

This commit addresses the following aspects of eventfd:
* make read() and write() atomic in respect to each other
* POLLIN after creating eventfd with initval != 0 shall be set
* blocking and nonblocking modes shall have the same effect on poll()
* add support for POLLOUT

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>
1 file changed