Make sure keys are generated with the right source of randomness
Before this change, bolt_get_random_data would fall back if getrandom(2) could not be used. The fallbacks can not ensure we have a good enough source of randomness for key creation. Therefore if we can not use the right source just fail, instead of (maybe) creating weak keys.
In reality this should have never happened on a modern enough system, i.e. one that has getrandom(2).
Closes #132 (closed)