Add slashguard feature

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

src/ui/console.c

@@ -2049,6 +2049,16 @@ cons_avatar_setting(void)
     prefs_free_string(pref);
 }
 
+void
+cons_slashguard_setting(void)
+{
+    if (prefs_get_boolean(PREF_SLASH_GUARD)) {
+        cons_show("Slashguard (/slashguard)       : ON");
+    } else {
+        cons_show("Slashguard (/slashguard)       : OFF");
+    }
+}
+
 void
 cons_show_connection_prefs(void)
 {