Motivation: The libcap user may want to associate some contextual
information for each capability (rather than with the object). For
example, the LCD microkernel uses metadata to track whether a page
referred to by a page capability has been mapped and where.

This metadata field *is not* carried over to child capabilities
during grant, and is NULL'd out when a capability is deleted from
a cspace (so when the slot is used again, the field starts out as
NULL).

Internals note: The cnode will start out with NULL metadata as well
when it is first used because we do a zalloc to create cnode tables.