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Minor NetBSD cleanups

- Add architecture check for NetBSD/ibmnws;
- Clean up resource limit initialization and mmap in small chunks;
- Test for SSE sysctl value when selecting bcopy variant.
sb-posix NetBSD fixups

- argument to mmap(2) is size_t, not unsigned;
- disable tests for fcntl(2)
- wrap utimes(2)
Initial support for threads on NetBSD/amd64.