BUFFER_A/B -> BATTLELINKMSGTYPE; link send/recv. task data names

Replaced the BUFFER_A and BUFFER_B constants with the BATTLELINKMSGTYPE enum, which also identifies the "controller becoming idle" message type.

Named the task data for the link send task and the link receive task. Added macros to functions related to those tasks, for legibility.

include/battle_controllers.h

@@ -66,9 +66,10 @@ enum {
 
 // Special arguments for Battle Controller functions.
 
-enum { // Values given to the emit functions to choose gBattleBufferA or gBattleBufferB
-    BUFFER_A,
-    BUFFER_B
+enum {
+   BATTLELINKMSGTYPE_ENGINE_TO_CONTROLLER     = 0, // gBattleBufferA
+   BATTLELINKMSGTYPE_CONTROLLER_TO_ENGINE     = 1, // gBattleBufferB
+   BATTLELINKMSGTYPE_CONTROLLER_BECOMING_IDLE = 2,
 };
 
 enum {