(prd_maxno): Variable's value is well-known constant P_MDUST + 2.

Useless, remove.  Users changed to check pchr's sentinel, because
that's clearer than comparing the index to P_MDUST or sizeof(pchr) /
sizeof(*pchr).

(pchr): Change sentinel's p_name and p_sname to NULL.  Sentinel
checks changed.

include/product.h

@@ -77,6 +77,4 @@ struct pchrstr {
 
 extern struct pchrstr pchr[P_MDUST + 2];
 
-extern int prd_maxno;
-
 #endif /* _PRODUCT_H_ */