cleanup julia_init options

moves all configuration options into jl_compileropts
compute the abspath/realpath of file paths in the config to avoid issues with chdir
simplify initialization / startup / embedded / task-switch api
code cleanup
remove extraneous gc root in start_task
allow the user to specify whether the image_file should be interpreted relative to JULIA_HOME or CWD
update embedding doc to be consistent with changes to julia_init
some more init cleanup, and comments
Merge pull request #9266 from JuliaLang/jn/init_opt_cleanup

cleanup julia_init options