Let ifmgmt.c take care of calling efree(), since it's the once which
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.
Provide registration mechanism for loaded images, so that we can e.g.
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.
Tear out old heap code, replace with code that simply allocates memory
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! :)