Separate initialization and finalization

Right now, _start initializes global state, calls main(), and then tears down global state.

This is fine for executable modules, but not for modules designed to be used as libraries.

For a library, I would like to initialize global state right after the library is loaded, and then tear down global state right before shutting the library down.

I am proposing separating the initialization part into a separate exported function (call it _init) and, similarly, splitting the finalization into a separate exported function (call it _fini).

Then _start becomes (pseudocode):

void _start() {
    _init();
    // set up arguments for main(), call main()
    _fini();
}

but people who intend to use the module as a library have the option to call _init() and _fini() directly.

Start by locating the current _start implementation and tracing how it initializes global state, calls main(), and tears that state down. Define the exported _init and _fini entry points so library users can call them independently, while _start preserves the existing sequence; done means both executable and library use cases are supported.