When the 16-bit segment registers are accessed using 32-bit instructions
the high order bytes are undefined on older CPUs.  We now explicitly
zero the high order bytes when snapshotting the CPU state.  This ensures
that the GDB stub reports consistent values for the segment registers.

This commit implements GDB over UDP.  Using UDP is more complex than
serial and has required some restructuring.

The GDB stub is now built using one or both of GDBSERIAL and GDBUDP
config.h options.

To enter the debugger, execute the gPXE shell command:
gdbstub <transport> [<options>...]

Where <transport> is "serial" or "udp".  For "udp", the name of a
configured network device is required:
gdbstub udp net0

The GDB stub listens on UDP port 43770 by default.

This commit also includes a test to ensure that single stepping works,
since continue, kill, detach, and single step all share code.

iSCSI hack.

reflect the fact that they allocate and deallocate user memory (i.e.
things reached through a userptr_t).

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.

that switches from line-oriented to byte-oriented partway through, such as
HTTP.

Add debug autocolourisation to TFTP.

copy_{to,from}_phys.

can't modify it).

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

TargetAddress from the login response, because we still need the old
address while we close the connection!

"Permission denied" as an error text to strerror().

abstraction.

determine iSCSI boot parameters.

Remove the now-totally-obsolete sockaddr_in field from tcp.h.