cryptsetup: craft a unique ID with the source device

If cryptsetup is called with a source device as argv[3], then craft the
ID for the password agent with a unique device path.

If possible "/dev/block/<maj>:<min>" is used, otherwise the original
argv[3] is used.

This enables password agents like petera [1] to provide a password
according to the source device. The original ID did not carry enough
information and was more targeted for a human readable string, which
is specified in the "Message" field anyway.

With this patch the ID of the ask.XXX ini file looks like this:
ID=cryptsetup:/dev/block/<maj>:<min>

[1] https://github.com/npmccallum/petera

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