Tidy up code

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.

Add debug autocolourisation to TFTP.

routing table entry just to fool ipv4.c any more.

defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.

list.

create_dhcp_packet() from pxe_preboot.c, after the dhcp_session is long
gone.

Expose the functions required by pxe_preboot.c

udp_connect() now follows the standard BSD sockets semantics and simply
sets the default address for outgoing packets; it doesn't filter incoming
packets.

Introduced struct sockaddr_tcpip, to simplify code that deals with
both IPv4 and IPv6 addresses.

Reorganised parts of tcpip.c and udp.c.

all servers.

Parameter request list is yet another option that ISC dhcpd ignores
unless it's in the main options block.

Fix logic error in parsing file and sname fields.

cycle.  :)

doesn't seem to be required by the RFCs, but at least ISC dhcpd and
ethereal refuse to recognise it otherwise.

packets.

(apart from the bad UDP checksum).

functions that return status codes.

Zero the DHCP packet buffer *before* creating the options structures
inside it.

parsed DHCP responses.

structure, and populating it with options.  This should allow us to
use the same basic options list for both DHCPDISCOVER and DHCPREQUEST,
plus making it much easier to set the non-constant parameters
(e.g. requested IP address) in request packets.

derived is a 16-bit big-endian field, but htype is only 8 bits wide.

is a DHCP option" data structures.

We need this code in order to be able to return a DHCP packet to a PXE NBP
which reflects options from our multiple sources (e.g. NVS and DHCP
server).  This is expensive, but necessary.  Having paid this cost, we may
as well try to use the same code to generate our DHCP request packets,
since the process is similar.

DHCP options block from the contents.