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.
queue functions. This includes a private queue.h header derived
from FreeBSD. It is simpler to just use our own header rather than
try to deal with macros that may or may not be present in various
queue.h incarnations.