cleanup julia_init options

this attempts to be more consistent and centralized about initializing julia paths (build_path, image_file, julia_home). this is in an attempt to make embedding julia easier. also, it should help with spawning child processes (by providing more reliable access to the parameters, without dependence of the working directory)

1. moves all configuration options into jl_compileropts
2. computes the abspath/realpath of file paths in the config to avoid issues with chdir causing files to end up in the wrong location

@tkelman @ihnorton @staticfloat
Yes, the current behavior was intended. Actually it would be better and simpler to also allow absolute paths for everything. I think the following cases are useful:

```
jl_init_with_image(julia_home_abs_path, image_abs_path);

jl_init_with_image(NULL, image_abs_path);

// if I don't know where julia is installed, but I want sys-foo.ji from within that install
jl_init_with_image(NULL, image_rel_path);
```

That last case happens if somebody builds julia plus a couple images, and then distributes the whole directory to several people who put it in different places.