Renames vars on pdas and removes ntnet_relays glob (#79595)

## About The Pull Request

- replaces the ntnet_relays global list with ``get_machines_by_type``
- renames ``transfer_access`` var on PDA to ``download_access`` &
``required_access`` to ``run_access`` to better describe what they do,
because on more than one ocassion I've confused myself with the two
terms and ended up doing something unintentional (see:
https://github.com/tgstation/tgstation/pull/74269)

## Why It's Good For The Game

Kills a useless global list and makes pda code a little less confusing.

## Changelog

Nothing player-facing.

code/modules/modular_computers/file_system/programs/airestorer.dm

@@ -7,7 +7,7 @@
 	size = 12
 	requires_ntnet = FALSE
 	usage_flags = PROGRAM_CONSOLE | PROGRAM_LAPTOP
-	transfer_access = list(ACCESS_RD)
+	download_access = list(ACCESS_RD)
 	available_on_ntnet = TRUE
 	tgui_id = "NtosAiRestorer"
 	program_icon = "laptop-code"