client: Add WL_MAP_ENTRY_ZOMBIE flag

Add a new map entry flag to indicate that the object received is valid,
but a zombie. Previously this relied on a fixed object pointer, but
future patches in this series will have map entries returning either
NULL, or a different structure type entirely, for zombie objects.

wl_object_is_zombie() now solely uses the new flag to determine whether
or not the object is a zombie.

[daniels: Extracted from Derek's bespoke-zombie patch as an intermediate
          step.]

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>