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
it seemed like the default expectation is that if the user gives a relative path, the expectation is that it is relative to cwd. that was also the old behavior in the repl, I just reduced the amount of code duplication and variation by calling abspath on all inputs so that they wouldn't be affected if the cwd changes later in the program execution.  the primary expectation is definitely that the user will construct an absolute path if they want anything special. I think a `julia_init(...)` parameter indicating whether the user wants the relative path interpreted with respect to the cwd, julia_home, or (in the future) libjulia would be appropriate for this