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

[arichardson]

Summary

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

> usr.sbin/ntp/libntpevent (all)

.../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);
~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Test Plan

Compiles and boots (RISCV64)

[arichardson]

ping?

[jhb]

Should queue(3) also be updated?

sys/sys/queue.h

93

Should perhaps add an _END entry into this table?

[arichardson]

In D27151#631501, @jhb wrote:

Should queue(3) also be updated?

The manpage uses NULL directly in examples. I'm not sure it makes sense to document _END there since it only exists for compatibility with other queue.h headers.

[arichardson]

update the doc comment

[jhb]

I think I had patched libevent in ntp at one point to fix the #ifdef's so it would define the right macro locally. :)

I do find the END macros made more sense with something like CIRCLEQ, but not with the lists, so having it be obscure is fine by me. I would perhaps alter the commit log to reflect this though by saying that we are providing these for compat with other queue.h headers since some software assumes it exists, but we are not encouraging their use. You can then mention the embedded libevent as an example of the software that assumes their existence.

[arichardson]

In D27151#632456, @jhb wrote:

I think I had patched libevent in ntp at one point to fix the #ifdef's so it would define the right macro locally. :)

I do find the END macros made more sense with something like CIRCLEQ, but not with the lists, so having it be obscure is fine by me. I would perhaps alter the commit log to reflect this though by saying that we are providing these for compat with other queue.h headers since some software assumes it exists, but we are not encouraging their use. You can then mention the embedded libevent as an example of the software that assumes their existence.

Sounds good, I'll make sure to update the commit message.