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

[a3f]

Since Linux commit torvalds/linux@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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[a3f]

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