Shorter NACK packets take less time to construct and spew out less
debug output, and there's a limit to how useful it is to send a
complete missing-block list anyway; if the loss rate is high then
we're going to have to retransmit an updated missing-block list
anyway.

Also add pretty debugging output to show the list of requested blocks.

slam_parse_multicast_address() was failing to strip the initial "/"
from the URI path.

Tested against the mini-slamd server located in contrib/mini-slamd
with a single client, on a lossy network.