Fix an xxx wrt picking libevent methods known-to-work

The short version is, "where we want to do it, we have nothing real to
chose from and we can't do it easily. Where it's easy to do, we have
no reason to do it yet."

src/common/compat_libevent.c

@@ -340,17 +340,21 @@ tor_check_libevent_version(const char *m, int server,
 
   version = tor_get_libevent_version(&v);
 
-  /* XXX Would it be worthwhile disabling the methods that we know
-   * are buggy, rather than just warning about them and then proceeding
-   * to use them? If so, we should probably not wrap this whole thing
-   * in HAVE_EVENT_GET_VERSION and HAVE_EVENT_GET_METHOD. -RD */
-  /* XXXX The problem is that it's not trivial to get libevent to change it's
-   * method once it's initialized, and it's not trivial to tell what method it
-   * will use without initializing it.  I guess we could preemptively disable
-   * buggy libevent modes based on the version _before_ initializing it,
-   * though, but then there's no good way (afaict) to warn "I would have used
-   * kqueue, but instead I'm using select." -NM */
-  /* XXXX022 revist the above; it is fixable now. */
+  /* It would be better to disable known-buggy methods than to simply
+     warn about them.  However, it's not trivial to get libevent to change it's
+     method once it's initialized, and it's not trivial to tell what method it
+     will use without initializing it.
+
+     If we saw that the version was definitely bad, we could disable all the
+     methods that were bad for that version.  But the issue with that is that
+     if you've found a libevent before 1.1, you are not at all guaranteed to
+     have _any_ good method to use.
+
+     As of Libevent 2, we can do better, and have more control over what
+     methods get used.  But the problem here is that there are no versions of
+     Libevent 2 that have buggy event cores, so there's no point in writing
+     disable code yet.
+  */
   if (!strcmp(m, "kqueue")) {
     if (version < V_OLD(1,1,'b'))
       buggy = 1;