qeueue.h: Add {SLIST,STAILQ,LIST,TAILQ}_END()

We provide these for compat with other queue.h headers since some software
assumes it exists (e.g. the libevent contrib code), but we are not
encouraging their use (NULL should be used instead).

This fixes the following warning (which should arguable be an error since
it results in a function call to an undefined function):

.../contrib/libevent/buffer.c:495:16: warning: implicit declaration of function 'LIST_END' is invalid in C99 [-Wimplicit-function-declaration]

cbent != LIST_END(&buffer->callbacks);
         ^

.../contrib/libevent/buffer.c:495:13: warning: comparison between pointer and integer ('struct evbuffer_cb_entry *' and 'int') [-Wpointer-integer-compare]

cbent != LIST_END(&buffer->callbacks);
~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D27151