Elevates (almost all) inventory variables from /carbon to /human (#96955)

## About The Pull Request

Moves all inventory slots but handcuffs/legcuffs (as those can be used
by xenos) from ``/carbon`` onto ``/human``, as xenos do not use any of
those inventory slots, only leaving them in use by humans. Their
presence on ``/carbon`` is an artifact of times when monkeys weren't
humans, and some of the slots had to be shared by all carbons.
In some places I've used ``get_item_by_slot`` rather than swapping
checks to ``ishuman`` for simplicity's sake, in some places it might not
be the most optimal solution but in cases like help act any other
solution would require a refactor of the whole (massive) proc.

## Why It's Good For The Game

Cleaner/more sensible code, one step closer to fully datumized
inventories.

## Changelog
🆑
refactor: Moved a lot of human-specific inventory code onto human mobs,
report if inventories break!
/🆑

code/modules/mod/mod_link.dm

@@ -277,8 +277,8 @@
 		call_link(user, mod_link)
 
 /obj/item/clothing/neck/link_scryer/proc/get_user()
-	var/mob/living/carbon/user = loc
-	return istype(user) && user.wear_neck == src ? user : null
+	var/mob/living/user = loc
+	return istype(user) && user.get_item_by_slot(ITEM_SLOT_NECK) == src ? user : null
 
 /obj/item/clothing/neck/link_scryer/proc/can_call()
 	var/mob/living/user = loc