Specify better method for adding seed, combining entropy.

1) When adding seed, run the PRF every KEY_LEN/2 bytes, not every
KEY_LEN bytes. That way, even a pessimally chosen seed (say, one
computed as a function of the current state) can only hurt the future
results so much, assuming they can't break the PRF.

2) When combining entropy, treat it as adding seed rather than XORing
it all together.
Implement new KEY_LEN/2-at-a-time seeding algorithm.

Make changes as needed to cause tests to pass.
Use add-seed algorithm to combine multiple entropy sources

Previously, when we had multiple entropy sources, we would just XOR
them together. This is okay under all likely sources, but it doesn't
defend quite so well against the full perversity of possible entropy
source perversity.
update TODO from entropy_combine branch
Test odd entropy-source outputs again.
Restore testing of disabling OTTERY_CPUCAP_RAND
Merge branch 'topic/entropy_combine'