Add FILE_API_CHECK to the PXEXT API so the NBP can query the
availability and status of the API.

This allows pxelinux to execute arbitrary gPXE commands.  This is
remarkably unsafe (not least because some of the commands will assume
full ownership of memory and do nasty things like edit the e820 map
underneath the calling pxelinux), but it does allow access to the
"sanboot" command.

s_PXENV_TFTP_READ_FILE.

Added reason for min value of 512 on s_PXENV_TFTP_OPEN::PacketSize.

the Etherboot implementation (i.e. don't do what Intel did).

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