Add slashguard feature

New command `/slashguard` tries to protect against typing ` /quit` by
not allowing a slash in the first 4 characters.

src/ui/ui.h

@@ -320,6 +320,7 @@ void cons_color_setting(void);
 void cons_os_setting(void);
 void cons_correction_setting(void);
 void cons_avatar_setting(void);
+void cons_slashguard_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);