Finish and polish new_game.c

Only one function is left undocumented. I tried to go into the assembly
to at least name it, but it clears fields in the save block that aren't
even documented yet.

include/player_pc.h

@@ -85,5 +85,7 @@ extern const struct MenuAction gMailboxMailOptions[];
 void ReshowPlayerPC(u8 taskId);
 void sub_816B31C(void);
 void Mailbox_ReturnToMailListAfterDeposit(void);
+void NewGameInitPCItems(void);
+
 
 #endif //GUARD_PLAYER_PC_H