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/wield.c

@@ -844,6 +844,7 @@ untwoweapon(void)
     return;
 }
 
+/* enchant wielded weapon */
 int
 chwepon(struct obj *otmp, int amount)
 {
@@ -897,7 +898,7 @@ chwepon(struct obj *otmp, int amount)
         if (otyp != STRANGE_OBJECT)
             makeknown(otyp);
         if (multiple)
-            encumber_msg();
+            (void) encumber_msg();
         return 1;
     } else if (uwep->otyp == CRYSKNIFE && amount < 0) {
         multiple = (uwep->quan > 1L);
@@ -914,7 +915,7 @@ chwepon(struct obj *otmp, int amount)
         if (otyp != STRANGE_OBJECT && otmp->bknown)
             makeknown(otyp);
         if (multiple)
-            encumber_msg();
+            (void) encumber_msg();
         return 1;
     }