rand/randfile.c: permit non-regular files in RAND_load_file.

Apparently applications rely on RAND_load_file's ability to work with
non-regular files, customarily with /dev/urandom, so that the ban was
not exactly appropriate.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5737)