kern/sched: Hide scheduler selection from C++

[des]

Summary

The scheduler selection interface uses names that are reserved words in
C++, causing problems for downstream projects that use C++ in the
kernel. Work around this by hiding the interface from C++ compilers
until we can come up with a better solution.

Fixes: ce38acee8d0b ("Add kern/sched_shim.c")
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.

[siderop1_netapp.com]

sys/sys/sched.h

287

What's the concern/issue with just renaming these members to something like classifier or whatnot?

[des]

sys/sys/sched.h

287

The entire scheduler API is generated by macros based on these fields. Changing class and throw here forces us to also rename sched_class() and sched_throw(), which have been part of the published scheduler API for years.

[siderop1_netapp.com]

sys/sys/sched.h

287

Thanks for the explanation.

[imp]

This is fine. The only other viable change would be to use class_ or something like that when C++ is defined, but that's a bit too ugly absent some better articulated use case. C++ in the kernel isn't officially supported, though I know of various people that have made it kinda work over the years.