src: ensure that openssl's PRNG is fully seeded

Ensure that OpenSSL has enough entropy (at least 256 bits) for its PRNG.

The entropy pool starts out empty and needs to fill up before the PRNG
can be used securely.

OpenSSL normally fills the pool automatically but not when someone
starts generating random numbers before the pool is full: in that case
OpenSSL keeps lowering the entropy estimate to thwart attackers trying
to guess the initial state of the PRNG.

When that happens, we wait until enough entropy is available, something
that normally should never take longer than a few milliseconds.

Fixes #7338.