Cleanup RAND_load_file,RAND_write_file

Document an internal assumption that these are only for use with files,
and return an error if not. That made the code much simpler.
Leave it as writing 1024 bytes, even though we don't need more than 256
from a security perspective.  But the amount isn't specified, now, so we
can change it later if we want.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3864)