export Core.JULIA_IMAGE_FILE

As discussed with @JeffBezanson and @jakebolewski, this adds `Core.JULIA_IMAGE_FILE` so that you have access in Julia to the image filename.

e.g. this is useful in pyjulia, or any other program that wants to use the `jl_init_with_image`, so that it can get the image-file path from Julia.
Now that I think about it, it doesn't seem to be of much value for these variables to be `const`. They aren't performance-critical, and if somebody overwrites them that's their own problem.

There is too much code in dump.c devoted to handling `JULIA_HOME`, and we should just get rid of all that and simply re-assign these variables on startup.