defined EntropyReseedLevel to tweak how entropy reseed is done.

Level None specify no entropy reseeding, so that a CPRG can be used
for debugging and testing purpose, where the quality and security of
random is not what we are after.

Other level allow to specify different speeds which are advisory at best.
pass EntropyReseedLevel to create and fork.
add a new cprgGenerateWithEntropy which allows to generate bytes which are
mixed with entropy straight after generation.

As the result is pure random, 2 calls to this function might not result in
the same result.
misc documentation change