unix: add Uvmexp and SysctlUvmexp for NetBSD

NetBSD has both struct uvmexp (used internally) and struct uvmexp_sysctl
(preferred for interfacing from user code). Expose the latter as type
Uvmexp with the same interface as on OpenBSD.

I am not sure if it makes sense to even have a name parameter for
SysctlUvmexp, since the only valid value can be "vm.uvmext2".

Change-Id: I40a743536c28e3fb7a54253768db35b1629fabfe
Reviewed-on: https://go-review.googlesource.com/c/sys/+/460156
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

unix/syscall_netbsd.go

@@ -110,6 +110,20 @@ func direntNamlen(buf []byte) (uint64, bool) {
 	return readInt(buf, unsafe.Offsetof(Dirent{}.Namlen), unsafe.Sizeof(Dirent{}.Namlen))
 }
 
+func SysctlUvmexp(name string) (*Uvmexp, error) {
+	mib, err := sysctlmib(name)
+	if err != nil {
+		return nil, err
+	}
+
+	n := uintptr(SizeofUvmexp)
+	var u Uvmexp
+	if err := sysctl(mib, (*byte)(unsafe.Pointer(&u)), &n, nil, 0); err != nil {
+		return nil, err
+	}
+	return &u, nil
+}
+
 func Pipe(p []int) (err error) {
 	return Pipe2(p, 0)
 }