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/dothrow.c

@@ -245,6 +245,7 @@ throw_obj(struct obj *obj, int shotlimit)
         }
         freeinv(otmp);
         throwit(otmp, wep_mask, twoweap, oldslot);
+        (void) encumber_msg();
     }
     g.m_shot.n = g.m_shot.i = 0;
     g.m_shot.o = STRANGE_OBJECT;