sys: dlist: Add sys_dnode_is_linked

The original implementation allows a list to be corrupted by list
operations on the removed node.  Existing code attempts to avoid this by
using external state to determine whether a node is in a list, but this
is fragile and fails when the state that holds the flag value is changed
after the node is removed, e.g. in preparation for re-using the node.

Follow Linux in invalidating the link pointers in a removed node.  Add
API so that detection of particpation in a list is available at the node
abstraction.

This solution relies on the following steady-state invariants:
* A node (as opposed to a list) will never be adjacent to itself in a
  list;
* The next and prev pointers of a node are always either both null or
  both non-null.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>