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
The command line interface and `jl_init_with_image` are not the same thing. `jl_init_with_image` called `jl_locate_sysimg` which simply joined the paths together, not using the CWD. The purpose of calling `jl_init_with_image` is to specify an image path that may be unrelated to the CWD or the main application's path.