MFC r304230:
Add two new macros, SLIST_CONCAT and LIST_CONCAT.

MFC r304239:
Bug 211013 reports that a write error to a UFS filesystem running
with softupdates panics the kernel.

PR: 211013

MFC r284915:
Make the system queue header file fully usable within C++ programs by
adding macros to define class lists.

This change is backwards compatible for all use within C and C++
programs. Only C++ programs will have added support to use the queue
macros within classes. Previously the queue macros could only be used
within structures.

The queue.3 manual page has been updated to describe the new
functionality and some alphabetic sorting has been done while
at it.

Differential Revision: https://reviews.freebsd.org/D2745
PR: 200827 (exp-run)

MFC r279241 and r279242:
Fix debugging macros.

MFC r279241 and r279242:
Fix debugging macros.