Add slashguard feature

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

tests/unittests/ui/stub_ui.c

@@ -453,6 +453,7 @@ void cons_tray_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)
 {