keytab: Add new extended operation to get a keytab.
This new extended operation allow to create new keys or retrieve
existing ones. The new set of keys is returned as a ASN.1 structure
similar to the one that is passed in by the 'set keytab' extended
Access to the operation is regulated through a new special ACI that
allows 'retrieval' only if the user has access to an attribute named
ipaProtectedOperation postfixed by the subtypes 'read_keys' and
'write_keys' to distinguish between creation and retrieval operation.
For example for allowing retrieval by a specific user the following ACI
is set on cn=accounts:
This ACI matches only if the service object hosts a new attribute named
ipaAllowedToPerform that holds the DN of the user attempting the
Reviewed-By: Nathaniel McCallum <firstname.lastname@example.org>