PHP-820: MONGO_METHOD cleanup: Split out MongoDB::selectCollection into a C-function.
Use php_mongodb_selectcollection for MongoDB::__get().
Use php_mongodb_selectcollection for MongoDB::createCollection().
Use php_mongodb_selectcollection for MongoDB::dropCollection().
Use php_mongodb_selectcollection for MongoDB::listCollections()/getCollectionNames()
Use php_mongodb_selectcollection for MongoCollection::get().
Use php_mongodb_selectcollection for MongoCollection::ensureIndex().
Use php_mongodb_selectcollection for MongoCollection::getIndexInfo().
Use php_mongodb_selectcollection for MongoClient::selectCollection().
Use php_mongodb_selectcollection for MongoDBRef::get().
Fixed ZTS mode.
Fixed tests - the type depends on mongo.native_long being set.
Fixed prototypes.
Integrate Hannes' comments.
Check return values for php_mongodb_getcollection.
Rename variables in the same way as __get and do a null check for php_mongodb_selectcollection.
Check for NULL values here.

This simplifies checks in other places, and it makes sure we don't
inadvertently dtor NULL pointers.
Merged pull request #534