The majority of the tools have already options (--echo-keys &
--keys-from-stdin) to deal with LUKS credentials, although there is no
way to automatically provide credentials. --keys-from-stdin is
suboptimal, because it is a usable solution only when there is just one
device to open, and no other input passed via stdin to the tool (like
the commands for guestfish).
To overcome this limitation, introduce a new --key option in tools:
* --key /dev/device:file:/filename/with/key
* --key /dev/device:string:the-actual-key
this way it is possible to pass all the credentials needed for the
specific devices to open, with no risk of conflict with stdin, and also
in a secure way (when using the "file" way).
On the technical side: this adds a new "key_store" API for the C tools,
making sure it is used only when needed. Partially mirror it also for
the OCaml tools, although there will be a conversion to the C API
because the decryption helpers used are in the common C parts.
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