encumbrance checks

I polymorphed into something wimpy and became overloaded or even
overtaxed so I dropped everything.  The status line still showed
overloaded or overtaxed until my next move.  That didn't happen in
3.6.x or 3.4.3 but I didn't pursue trying to figure out what caused
this misbehavior.

I wanted to add an encumber_msg() call to freeinv() but that would
cause message sequencing issues.  Instead, add a call to it in a
few places where items are leaving hero's inventory, particularly
for the chain of calls for dropping stuff.  I've left it off in a
bunch of other potential places.

Also add a few missing (void) casts where the return value of
existing encumber_msg() calls is being ignored.

src/invent.c

@@ -1065,7 +1065,7 @@ hold_another_object(struct obj *obj, const char *drop_fmt,
                 prinv(hold_msg, obj, oquan);
             /* obj made it into inventory and is staying there */
             update_inventory();
-            encumber_msg();
+            (void) encumber_msg();
         }
     }
     return obj;