libnvdimm/security, acpi/nfit: unify zero-key for all security commands

With zero-key defined, we can remove previous detection of key id 0 or null
key in order to deal with a zero-key situation. Syncing all security
commands to use the zero-key. Helper functions are introduced to return the
data that points to the actual key payload or the zero_key. This helps
uniformly handle the key material even with zero_key.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>