kernel/timeout: Add timeout remaining/expires APIs

Add tick-based (i.e. precision resistant) inspection APIs for kernel
timeouts visible via k_timer, k_delayed work and thread timeouts
(i.e. pended/sleeping threads).  These are each available in
"remaining" and "expires" variants returning time values relative to
current time and system start.  All have system calls where applicable
(i.e. everywhere but k_delayed_work, which is not a userspace API)

The pre-existing millisecond "remaining_get()" predicates for timer
and delayed work remain, but are expressed in terms of the newer
calls.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>