merge Alex's dual rng proposal with the isaac64 rng code and adjust

merge Alex's dual rng proposal with the isaac64 rng code and adjust

This is branched from Alex's hallu-rng-stability branch,
with two build corrections (detect.c, zap.c), and merged
with  the isaac64 branch that we have ready to go.

Alex's dual rng is supported by setting up the array
of multiple isaac64 contexts.

I stuck with Alex's approach of passing the rng function
name around as the parameter (rng or rn2_on_display_rng)
for the new additional parameter needed for
set_random(), init_random(), reseed_random(),
and init_isaac64().