ZFS: Request credentials only for selected pools
ZFS: Request credentials only for selected pools

This change introduces more fine-grained requestEncryptionCredentials.
While previously when requestEncryptionCredentials = true, the
credentials for all imported pools and all datasets in these imported
pools were requested, it is now possible to select exactly the pools and
datasets for which credentials should be requested.

It is still possible to set requestEncryptionCredentials = true, which
continues to act as a wildcard for all pools and datasets, so the change
is backwards compatible.

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