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/plugin.h

@@ -46,7 +46,8 @@ extern void plugin_unload_all ();
 extern void plugin_reload_name (const char *name, int argc, char **argv);
 extern void plugin_init (int auto_load, int argc, char *argv[]);
 extern void plugin_end ();
-extern struct t_hdata *plugin_hdata_plugin_cb (void *data,
+extern struct t_hdata *plugin_hdata_plugin_cb (const void *pointer,
+                                               void *data,
                                                const char *hdata_name);
 extern int plugin_add_to_infolist (struct t_infolist *infolist,
                                    struct t_weechat_plugin *plugin);