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/allmain.c

@@ -360,7 +360,7 @@ moveloop_core(void)
            inventory may have changed in, e.g., nh_timeout(); we do
            need two checks here so that the player gets feedback
            immediately if their own action encumbered them */
-        encumber_msg();
+        (void) encumber_msg();
 
 #ifdef STATUS_HILITES
         if (iflags.hilite_delta)