* get rid of the pointless fences
* make seed_len 16 instead of 32, which is accurate since it was
already padding the rest anyway; now we do 1 pad instead of 2.
* secureZero to clear the AT_RANDOM auxval
* add a flag root source files can use to disable the start code. This
is in case people want to opt out of the initialization when they
don't depend on it.