The vs_alloc_slot function allocates a new slot for storing a pointer to data specific to a certain VirtualServer*. The returned slot number may be used in subsequent vs_set_data and vs_get_data calls. The returned slot number is valid for any VirtualServer*.
The value of the pointer (which may be returned by a call to vs_set_data) defaults to NULL for every VirtualServer*.
int vs_alloc_slot(void);
A slot number on success, or -1 on failure.