refine PR #1135 - encumbrance messages

When picking up multiple items (or looting multiple items) in one
operation, only show encumbrance for the first item that causes that
to change instead of complaining about non-zero encumbrance on every
item.  The very first item is treated as if it caused a change, so
you get "you have a little trouble lifting <obj>" even if you were
already burdened, same as before.

Only lightly tested.

include/decl.h

@@ -775,6 +775,8 @@ struct instance_globals_p {
 
     /* pickup.c */
     boolean picked_filter;
+    int pickup_encumbrance; /* when picking up multiple items in a single
+                             * operation, encumbrance after previous item */
 
     /* pline.c */
     unsigned pline_flags;