Use named function types for friend_requests callbacks.

Also:
* `#define` must be scoped. If it's used outside a scope, it must be
  defined outside that scope (global `#define` does not need a matching
  `#undef`).

toxcore/friend_requests.c

@@ -32,20 +32,20 @@
 
 #include "util.h"
 
+#define MAX_RECEIVED_STORED 32
+
 struct Friend_Requests {
     uint32_t nospam;
-    void (*handle_friendrequest)(void *, const uint8_t *, const uint8_t *, size_t, void *);
+    fr_friend_request_cb *handle_friendrequest;
     uint8_t handle_friendrequest_isset;
     void *handle_friendrequest_object;
 
-    int (*filter_function)(const uint8_t *, void *);
+    filter_function_cb *filter_function;
     void *filter_function_userdata;
     /* NOTE: The following is just a temporary fix for the multiple friend requests received at the same time problem.
      * TODO(irungentoo): Make this better (This will most likely tie in with the way we will handle spam.)
      */
 
-#define MAX_RECEIVED_STORED 32
-
     uint8_t received_requests[MAX_RECEIVED_STORED][CRYPTO_PUBLIC_KEY_SIZE];
     uint16_t received_requests_index;
 };
@@ -63,16 +63,15 @@ uint32_t get_nospam(const Friend_Requests *fr)
 
 
 /* Set the function that will be executed when a friend request is received. */
-void callback_friendrequest(Friend_Requests *fr, void (*function)(void *, const uint8_t *, const uint8_t *, size_t,
-                            void *), void *object)
+void callback_friendrequest(Friend_Requests *fr, fr_friend_request_cb *function, void *object)
 {
     fr->handle_friendrequest = function;
     fr->handle_friendrequest_isset = 1;
     fr->handle_friendrequest_object = object;
 }
 
 /* Set the function used to check if a friend request should be displayed to the user or not. */
-void set_filter_function(Friend_Requests *fr, int (*function)(const uint8_t *, void *), void *userdata)
+void set_filter_function(Friend_Requests *fr, filter_function_cb *function, void *userdata)
 {
     fr->filter_function = function;
     fr->filter_function_userdata = userdata;

toxcore/friend_requests.h

@@ -48,11 +48,13 @@ typedef void fr_friend_request_cb(void *, const uint8_t *, const uint8_t *, size
  */
 void callback_friendrequest(Friend_Requests *fr, fr_friend_request_cb *function, void *object);
 
+typedef int filter_function_cb(const uint8_t *, void *);
+
 /* Set the function used to check if a friend request should be displayed to the user or not.
  * Function format is int function(uint8_t * public_key, void * userdata)
  * It must return 0 if the request is ok (anything else if it is bad.)
  */
-void set_filter_function(Friend_Requests *fr, int (*function)(const uint8_t *, void *), void *userdata);
+void set_filter_function(Friend_Requests *fr, filter_function_cb *function, void *userdata);
 
 /* Sets up friendreq packet handlers. */
 void friendreq_init(Friend_Requests *fr, Friend_Connections *fr_c);