Improve sq network wkd publish's output.

- Add a header to a list so that it doesn't run into the previous
    list.  Consider:

    ```
    $ sq network wkd publish --domain example.org  /tmp/example.org --cert 9889EAA8B493129489EF7D235CE7215F115DA8A6
    Note: Skipping the following certificates because they don't have a valid self-signed user ID in example.org:
     - 9889EAA8B493129489EF7D235CE7215F115DA8A6
     - F57D5B402CEA11F8F22228A64AC1A5F0C16EE42D
       - alice (UNAUTHENTICATED)
       - updated

    1 update applied.
    ```

src/commands/network.rs

@@ -1372,7 +1372,13 @@ pub fn dispatch_wkd(mut sq: Sq, c: cli::network::wkd::Command)
 
             // Reports on certificate updates, or the lack thereof.
             let sq_ref = &sq;
+            let mut printed_updating = false;
             let mut status = |cert: &Cert, msg: &str| {
+                if ! printed_updating {
+                    printed_updating = true;
+                    qprintln!("Updating:");
+                }
+
                 qprintln!(initial_indent = " - ", "{}", cert.fingerprint());
                 qprintln!(initial_indent = "   - ", "{}",
                           sq_ref.best_userid(&cert, false));