adds constants, cleans up callstd and msgbox code

* adds some constants for standard script names and messagebox types
* replaces some callstd calls with msgbox calls where equivalent
* replaces msgbox and callstd calls using raw digits with versions that use the constants

data/maps/SafariZone_Northwest/scripts.inc

@@ -2,6 +2,6 @@ SafariZone_Northwest_MapScripts:: @ 823D249
 	.byte 0
 
 SafariZone_Northwest_EventScript_23D24A:: @ 823D24A
-	msgbox SafariZone_Northwest_Text_2A542C, 2
+	msgbox SafariZone_Northwest_Text_2A542C, MSGBOX_NPC
 	end