lib{c,thr}: add kernel version guards on exterrctl

[brooks]

Summary

This allows userspace to run on a (somewhat) out of date kernel.

[brooks]

Compile tested only. Not calling exterrctl should be equivalent to it returning ENOSYS when the kernel doesn't support it.

[kevans]

Thanks!

[brooks]

I've now verified that a system with these patches applied, but the __FreeBSD_version bump rolled back boots fine and doesn't call exterrctl(). With the version bumped the calls are made.