kernel: userspace.c code cleanup

- Dumping error messages split from _k_object_validate(), to avoid spam
  in test cases that are expected to have failure result.

- _k_object_find() prototype moved to syscall_handler.h

- Clean up k_object_access() implementation to avoid double object
  lookup and use single validation function

- Added comments, minor whitespace changes

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/kernel.h

@@ -131,6 +131,8 @@ struct k_mem_partition;
  * function in kernel/userspace.c
  */
 enum k_objects {
+	K_OBJ_ANY,
+
 	/* Core kernel objects */
 	K_OBJ_ALERT,
 	K_OBJ_MSGQ,