WinPE's pxeboot.n12 takes the BufferLimit returned by gPXE (indicating
the size of gPXE's internal DHCP packet buffers) and erroneously passes
it in as BufferSize (indicating the size of pxeboot.n12's DHCP packet
buffer).  If these don't match, then pxeboot.n12 ends up instructing gPXE
to overwrite parts of its data segment.

Change gPXE's internal DHCP packet buffers to be exactly
sizeof(BOOTPLAYER_t) bytes to work around this problem.

stored DHCP packets.

PXENV_GET_CACHED_INFO.  NTLDR does this.

PXENV_GET_CACHED_INFO.  If we dont do this, Altiris' NBP screws up; it
relies on being able to grab pointers to each of the three packets and
then read them at will later.

s_PXENV_GET_CACHED_INFO, because this field doesn't exist in earlier
versions of the PXE spec, and some callers don't allocate space for it.

by PXE and NBI) and UNDI packets (as used by undinet and UNDI).

Implement PXENV_RESTART_TFTP.

Updated PXE API dispatcher to use copy_{to,from}_user, and moved to
arch/i386 since the implementation is quite architecture-dependent.
(The individual PXE API calls can be largely
architecture-independent.)

PXE code now compiles without errors (though it won't actually work).