This is an implementation detail, but it saves an indirection

syscall.Close is not safe to run twice. Reused fds cause random things
to break.

Test Lseek together with FOPEN_DIRECT_IO, and document this use-case
in directio_test.go

When embedding DefaultOperations, a single node type that does not
support an operation should cause the opcode to never be issued again,
which is what ENOSYS could potentially do.