net: sockets: Rework poll to support ioctl helper commands

This commit reworks socket poll implementation to support multiple
socket implementations.

To achieve that, two ioctl poll helper requests were added:
ZFD_IOCTL_POLL_PREPARE and ZFD_IOCTL_POLL_UPDATE. The poll
implementation calls ioctl with these requests for each socket
requested in the fds table.

The first request is responsible for preparing k_poll_event objects
for specific socket. It can request to skip waiting in k_poll by
returning EALREADY through errno.

The latter request is responsible for processing outcome of k_poll for
each socket. It can request to retry the k_poll by returning EAGAIN
through errno.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>