core: Mark exit() calls with status codes

Also, exit with 128+n for signal exit n, as is The Unix Way.
core: Remove the nextTick for running the main file

Not necessary, since we can handle the error properly on the first tick
now, even if there are event listeners, etc.

Additionally, this removes the unnecessary "_needTickCallback" from
startup, since Module.loadMain() will kick off a nextTick callback right
after it runs the main module.

Fix #4856