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.

Allow nil FileHandles.

Change DefaultOperations to be for a ReadOnly in-memory filesystem.

The fd is set on creation, and never written. We rely on the kernel
and on fileEntry.wg to synchronize such that RELEASE always happens
after read/write has completed.