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/ball.c

@@ -34,7 +34,7 @@ ballrelease(boolean showmsg)
         /* [this used to test 'if (uwep != uball)' but that always passes
            after the setuwep() above] */
         freeinv(uball); /* remove from inventory but don't place on floor */
-        encumber_msg();
+        (void) encumber_msg();
     }
 }
 
@@ -975,7 +975,6 @@ litter(void)
                     otense(otmp, "fall"));
                 freeinv(otmp);
                 hitfloor(otmp, FALSE);
-                encumber_msg(); /* drop[xyz]() probably ought to to this... */
             }
         }
     }