Apparently Haiku has a misbehaving /dev/urandom.
While we're at it, simplify the function signature to completely succeed
or completely fail and make sure the caller checks the result.
Reported-by: Alexander von Gluck IV <kallisti5@unixzen.com>
Nitpicked-by: Aaron Jones <aaronmdjones@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>