nodefs: implement the futimens(3) glibc function

This function is neither provided by the syscall package nor x/sys/unix.

fuse/nodefs/syscall.go

+package nodefs
+
+import (
+	"unsafe"
+	"syscall"
+)
+
+// futimens - futimens(3) calls utimensat(2) with "pathname" set to null and
+// "flags" set to zero
+func futimens(fd int, times *[2]syscall.Timespec) (err error) {
+	_, _, e1 := syscall.Syscall6(syscall.SYS_UTIMENSAT, uintptr(fd), 0, uintptr(unsafe.Pointer(times)), uintptr(0), 0, 0)
+	if e1 != 0 {
+		err = syscall.Errno(e1)
+	}
+	return
+}