Add a few more missing initialization-check tests
Intel CPUID support to detect SSE2/SSE3/RDRAND

Not yet actually used
SIMD implementations turn themselves off on non-SIMD intel CPUs
Make ottery_memclear non-static.
Build chacha_krovetz in sse3 and sse2 flavors; detect the right one to use
Unit test for selecting PRF by name or CPU capabilities
Add some make targets to figure out coverage properly

(Now that we're building two versions of chacha_krovetz.c, we need
to be a little smarter than gcov -o src src/*.c.)
Implement support for multiple entropy sources

This includes including RDRAND on recent intel chips.
fixup! Implement support for multiple entropy sources
fixup! Implement support for multiple entropy sources
Tweak the TODO slightly
Rearrange TODO