bpo-31927: Fix bugs in socketmodule.c on NetBSD and other issues. (#4235)

* Fix compilation of the socket module on NetBSD 8.
* Fix the assertion failure or reading arbitrary data when parse
  a AF_BLUETOOTH address on NetBSD and DragonFly BSD.
* Fix other potential errors and make the code more reliable.

Misc/NEWS.d/next/Library/2017-11-02-18-26-40.bpo-31927.40K6kp.rst

+Fixed compilation of the socket module on NetBSD 8.  Fixed assertion failure
+or reading arbitrary data when parse a AF_BLUETOOTH address on NetBSD and
+DragonFly BSD.

Modules/socketmodule.h

@@ -80,7 +80,10 @@ typedef int socklen_t;
 #endif
 
 #ifdef HAVE_LINUX_CAN_H
-#include <linux/can.h>
+# include <linux/can.h>
+#else
+# undef AF_CAN
+# undef PF_CAN
 #endif
 
 #ifdef HAVE_LINUX_CAN_RAW_H