bounds checking by doname() and xname()

Try harder to prevent buffer overflow when formatting objects.
I don't have any test cases where overflow has been happening so
don't really know whether this works reliably.  And it doesn't try
to check prefix construction by doname().  [Yet?]

include/decl.h

@@ -1085,6 +1085,12 @@ struct instance_globals_x {
     /* mkmaze.c */
     int xmin, xmax; /* level boundaries x */
 
+    /* objnam.c */
+    char *xnamep; /* obuf[] returned by xname(), for use in doname() for
+                   * bounds checking; differs from xname() return value
+                   * due to reserving PREFIX bytes at start and possibly
+                   * skipping leading "the " after constructing result */
+
     /* sp_lev.c */
     coordxy xstart, xsize;