Clearing the LOADED flag actually prevents users from doing clever things
such as loading an image, then loading a patch image, then executing the
first image.  (image_exec() checks for IMAGE_LOADED, so this sequence of
operations will fail if the LOADED flag gets cleared.)

This reverts commit 14c080020f.

Loading an image may overwrite part or all of any previously-loaded
images, so we should clear the LOADED flag for all images prior to
attempting to load a new image.

executing any image, not just a script.  (This will enable pxelinux to
use relative URIs, should it wish to.)

Some shuffling around of the image management code; this needs tidying up.

allows us to avoid execution loops without having to hack around the
image registration order.

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

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.

returns the allocated buffer.

refer to them by name from the command line, or build them into a
multiboot module list.

Use setting image->type to disambiguate between "not my image" and "bad
image"; this avoids relying on specific values of the error code.

for use by malloc().

This breaks the image-loading code (which previously used the heap to
allocate the buffer for downloading the image), but that's not a major
concern since I'm going to tear out all the image formats within the next
couple of days anyway.  Byebye, NBI!  :)