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

Various combinations of options 43.6, 43.7 and 43.8 dictate which
servers we send Boot Server Discovery requests to, and which servers
we should accept responses from.  Obey these options, and remove the
explicit specification of a single Boot Server from start_pxebs() and
dependent functions.

The DHCP client code now implements only the mechanism of the DHCP and
PXE Boot Server protocols.  Boot Server Discovery can be initiated
manually using the "pxebs" command.  The menuing code is separated out
into a user-level function on a par with boot_root_path(), and is
entered in preference to a normal filename boot if the DHCP vendor
class is "PXEClient" and the PXE boot menu option exists.

Add yet another ugly hack to iscsiboot.c, this time to allow the user to
inhibit the shutdown/removal of the iSCSI INT13 device (and the network
devices, since they are required for the iSCSI device to function).

On the plus side, the fact that shutdown() now takes flags to
differentiate between shutdown-for-exit and shutdown-for-boot means that
another ugly hack (to allow returning via the PXE stack on BIOSes that
have broken INT 18 calls) will be easier.

I feel dirty.

Add ability for network devices to flag link up/down state to the
networking core.

Autobooting code will now wait for link-up before attempting DHCP.

IPoIB reflects the Infiniband link state as the network device link state
(which is not strictly correct; we also need a succesful IPoIB IPv4
broadcast group join), but is probably more informative.

Some shuffling around of the image management code; this needs tidying up.

iSCSI hack.

other protocol won't work at the moment.

will enable us to cascade async operations, which is necessary in order to
properly support DNS.  (For example, an HTTP request may have to redirect
to a new location and will have to perform a new DNS lookup, so we can't
just rely on doing the name lookup at the time of parsing the initial
URL).

Anything other than HTTP is probably broken right now; I'll fix the others
up asap.

returns the allocated buffer.

"kernel".