shim for require.main.filename() that works in as many environments as possible

 require.main.filename is great for figuring out the entry point for the
 current application. This can be combined with a module like pkg-conf
 to, as if by magic, load top-level configuration.
 .
 Unfortunately, require.main.filename sometimes fails when an application
 is executed with an alternative process manager, e.g., iisnode.
 .
 require-main-filename is a shim that addresses this problem.
 .
 Node.js is an event-based server-side JavaScript engine.