Optionally disable system RNG

[jasondavies]

In order to make it easier for participants to follow "Eliminating the possibility of backdoors with high probability", it would be good if the system RNG could be disabled so that only user-provided entropy is used.

The user would then be able to record the input entropy in randomly-chosen N-1 of N compute runs and verify that no tampering took place using alternative software and environments.

Something like --disable-rng or --without-rng?

Since measuring entropy is kind of difficult/onerous, I'm tempted to say that only the most basic checks should be used (number of input characters), and leave the rest up to the user?