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.

src/decl.c

@@ -675,6 +675,7 @@ const struct instance_globals_p g_init_p = {
     0,       /* perm_invent_toggling_direction */
     /* pickup.c */
     FALSE, /* picked_filter */
+    0, /* pickup_encumbrance */
     /* pline.c */
     0U, /* pline_flags */
     UNDEFINED_VALUES, /* prevmsg */