fscryptctl: add support for adding key by serial (ID)

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Since Linux commit https://github.com/torvalds/linux/commit/93edd392ca ("fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY"), it's possible to pass the key ID of a "fscrypt-provisioning" key that Linux should retrieve the raw key material from instead of passing it directly from userspace.

This is useful to add fscrypt keys after unmounting and re-mounting. It would also prove useful should additional key types like trusted keys be allowed in future.

Thus add a new --serial parameter to add_key to facilitate this. --serial was chosen over --id to avoid confusion with the KEY_IDENTIFIER used in the remove_key, key_status and set_policy documentation, which it is not interchangeable with.

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This is PR is applicable regardless of my patch for adding fscrypt support for trusted keys.

Should a revised version of that patch be applied, I'll create a new pull request to adjust the documentation here appropriately. There is no code change necessary however, because the API used for fscrypt-provisioning keys is reused.

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