Prevent external_acl.cc "inBackground" assertion on queue overloads.

The enqueue check for external ACL lookups was inconsistent with the final
queue length check in ExternalACLLookup::Start(). The former allowed adding to
the already full (but not yet overflowing) queue while the latter
rightfully(?) asserted that the queue should not overflow.

TODO: Should the fatal queue_size check be relaxed to use 2*n_running logic,
used by some other seemingly similar checks?

TODO: There are approximately ten places where we check queue sizes against
n_running or n_active. Many of those checks are slightly different. This
inconsistency should be removed, probably by adding a few standard methods
for a few types of checks that are actually needed.

=== modified file 'src/external_acl.cc'
--- src/external_acl.cc	2012-09-04 09:10:20 +0000
+++ src/external_acl.cc	2013-01-11 23:19:07 +0000
@@ -822,41 +822,41 @@
         }
 
         entry = static_cast<external_acl_entry *>(hash_lookup(acl->def->cache, key));
 
         external_acl_entry *staleEntry = entry;
         if (entry && external_acl_entry_expired(acl->def, entry))
             entry = NULL;
 
         if (entry && external_acl_grace_expired(acl->def, entry)) {
             // refresh in the background
             ExternalACLLookup::Start(ch, acl, true);
             debugs(82, 4, HERE << "no need to wait for the refresh of '" <<
                    key << "' in '" << acl->def->name << "' (ch=" << ch << ").");
         }
 
         if (!entry) {
             debugs(82, 2, HERE << acl->def->name << "(\"" << key << "\") = lookup needed");
             debugs(82, 2, HERE << "\"" << key << "\": entry=@" <<
                    entry << ", age=" << (entry ? (long int) squid_curtime - entry->date : 0));
 
-            if (acl->def->theHelper->stats.queue_size <= (int)acl->def->theHelper->childs.n_active) {
+            if (acl->def->theHelper->stats.queue_size < (int)acl->def->theHelper->childs.n_active) {
                 debugs(82, 2, HERE << "\"" << key << "\": queueing a call.");
                 ch->changeState(ExternalACLLookup::Instance());
                 debugs(82, 2, HERE << "\"" << key << "\": return -1.");
                 return ACCESS_DUNNO; // expired cached or simply absent entry
             } else {
                 if (!staleEntry) {
                     debugs(82, DBG_IMPORTANT, "WARNING: external ACL '" << acl->def->name <<
                            "' queue overload. Request rejected '" << key << "'.");
                     external_acl_message = "SYSTEM TOO BUSY, TRY AGAIN LATER";
                     return ACCESS_DUNNO;
                 } else {
                     debugs(82, DBG_IMPORTANT, "WARNING: external ACL '" << acl->def->name <<
                            "' queue overload. Using stale result. '" << key << "'.");
                     entry = staleEntry;
                     /* Fall thru to processing below */
                 }
             }
         }
     }