Do not show the keyserver setting with gnupg 2.1.0.
* src/settingsdlg.c (_SettingsDlg): Add field gnupg21. (settings_dlg_init): Init new field. (settings_dlg_constructor): Do not show the keyserver option with gnupg 2.1. (update_show_advanced_options): Skip the keyserver option with 2.1 (keyserver_selected_from_list_cb): Ditto. (check_default_keyserver): Ditto. (load_settings, save_settings): Ditto.