utility classes for symlinks and regular files without backing store.

The node can always call into the file handle if necessary, but the
reverse limits options for implementors.

Instead, define InodeEmbedder, an interface that can only be satisfied
by a struct that embeds Inode.

This is the prevalent style for FUSE libraries in Go.

Thanks to Ryan Lamore (ryanlamore) for spotting this

FOPEN_CACHE_DIR was added in Linux 4.19:

	https://git.kernel.org/linus/6433b8998a

Provide InodeLink/InodeEmbed which implements linking machinery.

This is unlikely to be useful, but makes the API more uniform.

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