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

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.

after a call to load().

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.