Move pxe_boot() code to pxe_image.c

This bug found by setting read and write watchpoints at NULL in bochs.

Shut down before jumping to the multiboot image.

do it.

set_fbms() will also update the E820 hidden region.

to start on a non-page boundary.  Make life safer by rounding out our
edited regions.

Add IMAGE_PXE to use the new image format framework.  "kernel pxelinux.0"
now works.

don't overwrite the parameter block until *after* the debug code.

clue what the "previous" interrupt handler will do, which could range
from "just an iret" to "disable the interrupt"), and that means that
we have to take responsibility for ACKing all interrupts.  Joy.

allocation.

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

order so we can just use buffer->fill.

if we don't.

address, to work around OS bugs.

took out the contract to eventually call efree() when it called fetch().

Maintain the most recently loaded image at the start of the list, so that
imgautoselect() will pick it.

Add scripts as an image format (since it's trivial to do).