drivers: nsos: introduce nsos_poll_if_blocking() helper function

Introduce nsos_poll_if_blocking(), which replaces common code in:
 * nsos_accept_with_poll()
 * nsos_recvfrom_with_poll()

This will allow to introduce similar behavior for other blocking APIs in
subsequent commits.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>