Record and report the number of peers (calculated as the maximum
number of peers discovered for a block's segment at the time that the
block download is complete), and the percentage of blocks retrieved
from peers rather than from the origin server.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Avoid accidentally dereferencing a NULL cipher context pointer for
plaintext blocks (which are usually messages with a block length of
zero, indicating a missing block).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

Signed-off-by: Michael Brown <mcb30@ipxe.org>