Should consolidate to a single implementation of main()

As part of SERVER-5520, we should get down to a single implementation of main() that never returns, but calls _exit(), instead, as part of a move toward not executing static destructors.

Basically, we'd have a generic main:
int main(...) {
initMongoOrDie();
int returnCode = mongoMain(argc, argv);
_exit(returnCode);
}

And then programs would implement their own mongoMain().