XEP-0092: Add configuration option to choose whether to send OS name

`/os on|off` now let's one choose whether to include the OS name once
`/software` (XEP-0092) is ran on us.

src/ui/ui.h

@@ -318,6 +318,7 @@ void cons_inpblock_setting(void);
 void cons_statusbar_setting(void);
 void cons_winpos_setting(void);
 void cons_color_setting(void);
+void cons_os_setting(void);
 void cons_show_contact_online(PContact contact, Resource *resource, GDateTime *last_activity);
 void cons_show_contact_offline(PContact contact, char *resource, char *status);
 void cons_theme_properties(void);