systm: Annotate copyin() and related functions with __result_use_check

[markj]

Summary

Now that all in-tree callers check for errors (or cast them away), we
can ask the compiler to check that new code does the same. This was
prompted by SA-23:18.nfsclient, which was caused by missing error
handling. This change is a weak mitigation since code can easily fail
to propagate error handling to the right place, but it's better than
nothing.

[markj]

There are three more patches in review that need to be committed before this can land.

I'll also request an exp-run to see if any kmod ports fall over. I tried a few popular ones already and didn't hit any build failures.