core, plugins: abort upgrade immediately if any upgrade file fails to be written

Detail of changes:

- the save of upgrade files in plugins is now done as soon as the "upgrade"
  signal is received, and not when the plugin is unloaded (it was too late to
  detect any problem and prevent the upgrade to happen)
- if the write of an upgrade file fails, the signal callback in plugin now
  returns WEECHAT_RC_ERROR and WeeChat checks this code to stop the upgrade as
  soon as this return code is received
- a new flag is added in plugin structure: unload_with_upgrade, it is set to 1
  before unloading all plugins when upgrade will happen (all *.upgrade files
  are then already successfully written).

src/plugins/irc/irc-buffer.c

@@ -199,7 +199,7 @@ irc_buffer_close_cb (const void *pointer, void *data,
                                              IRC_SERVER_OPTION_AUTOJOIN_DYNAMIC)
                 && ptr_server->is_connected
                 && !irc_signal_quit_received
-                && !irc_signal_upgrade_received)
+                && !weechat_irc_plugin->unload_with_upgrade)
             {
                 irc_join_remove_channel_from_autojoin (ptr_server,
                                                        ptr_channel->name);