Refactoring MONGO_METHOD calls in mongoclient.c and mongo.c

https://jira.mongodb.org/browse/PHP-820 and related sub-tasks
If we use 0 and 1, we end up checking for that instead of `== FAILURE` or `== SUCCESS`. I was following what @bjori used in the recent code for write batches. I agree that it sucks that PHP defines these as they do, but the names themselves seem more clear than using numeric codes.