Add a copy of OpenBSD's sys/queue.h as tor_queue.h

There are as many divergent implementations of sys/queue.h as there
are operating systems shipping it, it would seem.  They have some code
in common, but have drifted apart, and have added other stuff named
differently.  So I'm taking a relatively sane one, and hoping for the
best.

I'm taking OpenBSD's in particular because of the lack of external
dependencies, the presence of a CIRCLEQ (we could use one of those in
places), and the liberal licensing terms.

I'm naming the file tor_queue.h, since historically we've run into
trouble having headers with the same names as system headers (log.h,
for example.)

LICENSE

@@ -70,6 +70,35 @@ under the following license:
  * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
  * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
  * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+===============================================================================
+src/ext/tor_queue.h is licensed under the following license:
+
+ * Copyright (c) 1991, 1993
+ *      The Regents of the University of California.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
 
 ===============================================================================
 src/config/geoip is licensed under the following license:

changes/bsd_queue

@@ -0,0 +1,7 @@
+  o Code simplification and refactoring:
+    - Start using OpenBSD's implementation of queue.h, so that we don't
+      need to hand-roll our own pointer and list structures whenever we
+      need them. (We can't rely on a sys/queue.h, since some operating
+      systems don't have them, and the ones that do have them don't all
+      present the same extensions.)
+

src/ext/README

@@ -29,3 +29,11 @@ tinytest_macros.h
 
     A unit testing framework. https://github.com/nmathewson/tinytest
 
+tor_queue.h
+
+    A copy of sys/queue.h from OpenBSD.  We keep our own copy rather
+    than using sys/queue.h, since some platforms don't have a
+    sys/queue.h, and the ones that do have diverged in incompatible
+    ways.  (CIRCLEQ or no CIRCLEQ? SIMPLQ or STAILQ?)
+
+

src/ext/include.am

@@ -9,7 +9,8 @@ EXTHEADERS = \
   src/ext/tinytest.h	\
   src/ext/strlcat.c	\
   src/ext/strlcpy.c	\
-  src/ext/tinytest_macros.h
+  src/ext/tinytest_macros.h \
+  src/ext/tor_queue.h
 
 noinst_HEADERS+= $(EXTHEADERS)