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
`C_NULL` undefined, trying to write Julia in C?