Use this to simplify loopbackNode.

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.