Little TakeInventory refactoring.

Introduce AActor::TakeInventory, which unifies DoTakeInv from ACS and DoTakeInventory from Decorate, and AInventory::DepleteOrDestroy, which is extracted from the DoTakeInv core function, and use both where they're needed.
I don't know if the differences between DoTakeInv and DoTakeInventory were intentional, so I kept both behaviors.

src/g_shared/a_pickups.cpp

@@ -1131,6 +1131,32 @@ void AInventory::Destroy ()
 	if (SendItemDrop == this) SendItemDrop = NULL;
 }
 
+//===========================================================================
+//
+// AInventory :: DepleteOrDestroy
+//
+// If the item is depleted, just change its amount to 0, otherwise it's destroyed.
+//
+//===========================================================================
+
+void AInventory::DepleteOrDestroy ()
+{
+	// If it's not ammo or an internal armor, destroy it.
+	// Ammo needs to stick around, even when it's zero for the benefit
+	// of the weapons that use it and to maintain the maximum ammo
+	// amounts a backpack might have given.
+	// Armor shouldn't be removed because they only work properly when
+	// they are the last items in the inventory.
+	if (ItemFlags & IF_KEEPDEPLETED)
+	{
+		Amount = 0;
+	}
+	else
+	{
+		Destroy();
+	}
+}
+
 //===========================================================================
 //
 // AInventory :: GetBlend