core: add pointer in some callbacks (closes #406)

This pointer is the first argument received by callbacks, and the
existing argument "data" is now automatically freed by WeeChat when the
object containing the callback is removed.

With this new pointer, the linked list of callbacks in scripts has been
removed. This will improve speed of scripts (using a lot of hooks),
reduce memory used by scripts and reduce time to unload scripts.

Following functions are affected in the C API:

* exec_on_files
* config_new
* config_new_section
* config_new_option
* hook_command
* hook_command_run
* hook_timer
* hook_fd
* hook_process
* hook_process_hashtable
* hook_connect
* hook_print
* hook_signal
* hook_hsignal
* hook_config
* hook_completion
* hook_modifier
* hook_info
* hook_info_hashtable
* hook_infolist
* hook_hdata
* hook_focus
* unhook_all_plugin
* buffer_new
* bar_item_new
* upgrade_new
* upgrade_read

src/plugins/exec/exec-completion.c

@@ -32,7 +32,8 @@
  */
 
 int
-exec_completion_commands_ids_cb (void *data, const char *completion_item,
+exec_completion_commands_ids_cb (const void *pointer, void *data,
+                                 const char *completion_item,
                                  struct t_gui_buffer *buffer,
                                  struct t_gui_completion *completion)
 {
@@ -40,6 +41,7 @@ exec_completion_commands_ids_cb (void *data, const char *completion_item,
     char str_number[32];
 
     /* make C compiler happy */
+    (void) pointer;
     (void) data;
     (void) completion_item;
     (void) buffer;
@@ -69,5 +71,5 @@ exec_completion_init ()
 {
     weechat_hook_completion ("exec_commands_ids",
                              N_("ids (numbers and names) of executed commands"),
-                             &exec_completion_commands_ids_cb, NULL);
+                             &exec_completion_commands_ids_cb, NULL, NULL);
 }