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.

doc/fixes3-7-0.txt

@@ -1097,6 +1097,8 @@ changes to stair internals resulted in summoned Kops blockcading the stairs up
 	rather than intended stairs down
 dumplog's list of "major events" showed all logged events, not just major ones
 pickup via menu ignored player-specified count when picking up gold
+changes in encumbrance sometimes went unreported, leaving stale status line
+	info until hero's next move
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support