Added a pool argument to cbs creation so that cbss can share a pool. in particular, per-zone allocation cbss for the arena can share a single special block pool.

Copied from Perforce
 Change: 184469
 ServerID: perforce.ravenbrook.com

mps/code/arg.c

@@ -99,6 +99,11 @@ Bool ArgCheckdouble(Arg arg) {
   return TRUE;
 }
 
+Bool ArgCheckPool(Arg arg) {
+  CHECKD(Pool, arg->val.pool);
+  return TRUE;
+}
+
 
 ARG_DEFINE_KEY(args_end, Shouldnt);