kernel: timeout: detect inactive timeouts using dnode linked state

Whether a timeout is linked into the timeout queue can be determined
from the corresponding sys_dnode_t linked state.  This removes the need
to use a special flag value in dticks to determine that the timeout is
inactive.

Update _abort_timeout to return an error code, rather than the flag
value, when the timeout to be aborted was not active.

Remove the _INACTIVE flag value, and replace its external uses with an
internal API function that checks whether a timeout is inactive.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
diff --git a/include/kernel.h b/include/kernel.h
index fa18338..2edd113 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -873,9 +873,6 @@
 /* timeout has timed out and is not on _timeout_q anymore */
 #define _EXPIRED (-2)
 
-/* timeout is not in use */
-#define _INACTIVE (-1)
-
 struct _static_thread_data {
 	struct k_thread *init_thread;
 	k_thread_stack_t *init_stack;
@@ -1334,7 +1331,7 @@
 
 #define _K_TIMER_INITIALIZER(obj, expiry, stop) \
 	{ \
-	.timeout.dticks = _INACTIVE, \
+	.timeout.dticks = 0, \
 	.timeout.fn = _timer_expiration_handler, \
 	.wait_q = _WAIT_Q_INIT(&obj.wait_q), \
 	.expiry_fn = expiry, \