kernel: Introduce _THREAD_SLEEPING state bit

At the present time, Zephyr does has overlap between sleeping and
suspending. Not only should sleeping and suspended be orthogonal
states, but we should ensure users always employ the correct API.
For example, to wake a sleeping thread, k_wakeup() should be used,
and to resume a suspended thread, k_thread_resume() should be used.
However, at the present time k_thread_resume() can be used on a
thread that called k_sleep(K_FOREVER). Sleeping should have nothing
to do with suspension.

This commit introduces the new _THREAD_SLEEPING thread state along
with some prep-work to facilitate the decoupling of the sleeping and
suspended thread states.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>