kernel: add k_poll() API

k_poll() is similar to the POSIX poll() API in spirit in that it allows
a single thread to monitor multiple events without actively polling
them, but rather pending for one or more to become ready. Such events
can be a direct event, or kernel objects (currently only semaphores and
fifos).

When a kernel object being polled on is ready, it is not "given" to the
poller: the poller must then acquire it via the regular API for the
object (e.g. k_sem_take()). Only one thread can poll on a particular
object at one time. These restrictions mean that k_poll() is most
effective when a single thread monitors multiple events that are not
subject for contention. For example, being the sole reader on multiple
fifos, or the only thread being signalled by multiple semaphores, or a
combination of both.

Change-Id: I7035a9baf4aa016fb87afc5f5c0f5f8cb216480f
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>