Add "headless" tail queues and use them in place of the semi-circular

lists in sudoers.  Once the headless tail queue is built up it is
converted to a normal TAILQ.  This removes the last consumer of
list.c and list.h so those can now be removed.

plugins/sudoers/regress/parser/check_fill.c

@@ -43,7 +43,7 @@
 #define SUDO_ERROR_WRAP 0
 
 #include "missing.h"
-#include "list.h"
+#include "queue.h"
 #include "parse.h"
 #include "toke.h"
 #include "sudo_plugin.h"