Add evutil_secure_rng_set_urandom_device_file

This experimental function is needed for some seccomp2 hackery to
work, and should have no effect for systems that don't use it.
Finish a sentence
Avoid other RNG initialization FS reads when urandom file is specified
Oops; revert testing code
Really remove RNG seeds from the stack
Merge remote-tracking branch 'public/20_memclear' into patches-2.0