[dnsdist] Generate a key with cryptographically secure

[Y7n05h]

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Hi!

To implement https://github.com/PowerDNS/pdns/issues/9994,
we need to set TCP_FASTOPEN_KEY via setsockopt .
But how to generate a key?
I guess this key needs a random number that is cryptographically secure.
So we probably can't use rand() and random_engine from C or C++.
Perhaps /dev/urandom could be used, I found code using it.
I also noticed that there are some random number generation implementations
in the current code, but I'm not sure if they can be used in scenarios
where cryptographic security is required.

--
Best regards,

Y7n05h

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