DG Scripts bug fixes (#44)

* Increase ID space

DG Scripts uses tiny idspace that results in wacky bugs when the mud is
running too long.

* Overhaul script ids

All references to GET_ID(ch/obj) were removed and replaced by
char_script_id() and obj_script_id(), which don’t assign ids until they
are needed. The ch->id and obj->id variable names were changed to
script_id to prevent accidental errors for future programmers. This
change greatly increases how long the mud can run before it runs out of
ID space.

* Fix extraction count

This prevents an error log where it has over-counted the extractions
pending. It now behaves correctly when the same mob is %purge%’d or
extract_char()’d twice.

src/genobj.c

@@ -64,7 +64,7 @@ static int update_all_objects(struct obj_data *refobj)
     *obj = *refobj;
 
     /* Copy game-time dependent variables over. */
-    GET_ID(obj) = swap.id;
+    obj->script_id = swap.script_id;
     IN_ROOM(obj) = swap.in_room;
     obj->carried_by = swap.carried_by;
     obj->worn_by = swap.worn_by;