Make <sys/queue.h> usable with C++

[vangyzen]

Summary

Allow the queue macros to be used with C++ classes,
in addition to structures.

Test Plan

Done:
make buildworld on amd64
Pending:
make universe
ports exprun

[vangyzen]

This is an alternative to D2745.

[• hselasky]

sys/sys/_callout.h

44

You can also simply add:
"struct callout" here before the LIST_HEAD().

[• hselasky]

sys/sys/queue.h

147

I think it would be better if this macro was uppercased and took an argument like in D2745, else functionally the same.

[vangyzen]

Replaced by D2745.