Expose RAND_keep_random_devices_open()

[lukaslueg]

There is an issue open in cargo where open file descriptors to /dev/{u,}random leak to tests being executed by cargo. This is probably due to this commit in OpenSSL, which deliberately leaves the rand-source open in case we chroot somewhere where such sources are not available. A knob RAND_keep_random_devices_open() was introduced in the same commit, which would allow cargo to tell OpenSSL to close those file descriptors. AFAICS rust-openssl does not export this knob, which would probably solve rust-lang/cargo#6333

[lukaslueg]

Thanks