Work around a failure in Windows 10 jump lists.

We've had several reports that launching saved sessions from the
Windows 10 jump list fails; Changyu Li reports that this is because we
create those IShellLink objects with a command line string starting
with @, and in Windows 10 that causes the SetArguments method to
silently do the wrong thing.

windows/winjump.c

@@ -429,6 +429,10 @@ static IShellLink *make_shell_link(const char *appname,
     ret->lpVtbl->SetPath(ret, app_path);
 
     if (sessionname) {
+        /* The leading space is reported to work around a Windows 10
+         * behaviour change in which an argument string starting with
+         * '@' causes the SetArguments method to silently do the wrong
+         * thing. */
         param_string = dupcat(" @", sessionname, NULL);
     } else {
         param_string = dupstr("");