NetBSD doesn't have the right symbols defined in Go for mlockall support. The OS supports it just fine, but the definitions aren't present in Go. If someone wanted to they could add support XOR the values from `sys/mman.h` for `MCL_CURRENT | MCL_FUTURE` which is almost certainly `0x01 | 0x02` but we're not going to do that in code due to the maintenance of a one-off just for NetBSD. PR's welcome.
The `syscall` package has been frozen in favor of `x/sys`. As a result, all of the BSDs are supported and do have `mlockall(2)` support in current versions of Go.
