Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

When handling a redirection event, we need to close the existing
connection before opening the new connection.

Remove various 32-bit assumptions scattered throughout the codebase.
The code is still not necessarily 64-bit clean, but will at least
compile.

Add the definition of SLAM_MAX_BLOCKS_PER_NACK, which is roughly
equivalent to a TCP window size; it represents the maximum number of
packets that will be requested in a single NACK.

Note that, to keep the code size down, we still limit ourselves to
requesting only a single range per NACK; if the missing-block list is
discontiguous then we may request fewer than SLAM_MAX_BLOCKS_PER_NACK
blocks.

On any fast network, or with any driver that may drop packets
(e.g. Infiniband, which has very small RX rings), the traditional
usage of the SLAM protocol will result in enormous numbers of packet
drops and a consequent large number of retransmissions.

By adapting the client behaviour, we can force the server to act more
like a multicast TFTP server, with flow control provided by a single
master client.

This behaviour should interoperate with any traditional SLAM client
(e.g. Etherboot 5.4) on the network.  The SLAM protocol isn't actually
documented anywhere, so it's hard to define either behaviour as
compliant or otherwise.

In a SLAM NACK packet, if we run out of space to represent the
missing-block list, then indicate all remaining blocks as missing.
This avoids the need to wait for the one-second timeout before
receiving the blocks that otherwise wouldn't have been requested due
to running out of space.

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.