Carry entropy across invocations
Wen Tor starts, after seeding the CSPRNG, we should save some of its output to an "entropy" file. On later invocations, we should reseed from that file in addition to seeding from the OS, and then replace it with new CSPRNG output.
This is silly voodoo that might help us out if we're ever in a position where the OS has previously been well seeded but later is not. It can't make things worse, so long as RAND_add() is additive.
It's deferrable; #13696 (moved) is more important, assuming a non-broken OS.
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