Refactoring MONGO_METHOD calls in mongoclient.c and mongo.c

https://jira.mongodb.org/browse/PHP-820 and related sub-tasks
What are your thoughts on naming this `php_mongo_mongoclient_selectdb()`? That would be consistent with `php_mongo_db_init()` by having the module name incorporated into the function name, and make it clear where the function is located (if we care about that).

This may not be a concern since I renamed the first argument from `this` to `zlink`; with `this`, it was a bit confusing to know what object that should be at a glance.