OpenBSD throws compiler warnings that we can't reproduce on Linux, for
some reason.

Original patch from Dewey Hylton <dewey@hyltown.com>.

When the embedded image is a script, the unregister_image() performed by
image/script.c corrupts memory, since image/embedded.c omitted the call
to register_image().

This is the first bug fixed using Stefan Hajnoczi's gdb stub for gPXE.