Introduce credentials

These patches build on patch 9e2b2dc4133f65272a6d3c5dcb2ce63f8a87cae9 (CRED:
Introduce credential access wrappers) to wrap most of the accesses to a task's
credentials, whether by the task itself or by another task.

Not all are wrapped: under certain circumstances it is preferable or necessary
to deal with some accesses in other ways.  These will be dealt with by patches
that aren't in this set but are in linux-next.

The wrappings are there to make the implementation of copy-on-write credentials
easier.

David