Add support for offline files support, remote storage, and Async I/O force operations...

Add support for offline files support, remote storage, and Async I/O force operations to VFS

Offline files support and remote storage are for allowing communication with
backup and archiving tools that mark files moved to a tape library as offline.
We translate this info into corresponding CIFS offline file attribute and
mark an exported volume as remote storage.

Async I/O force is to allow selective redirection of I/O operations to asynchronous
processing in case it is viable at VFS module discretion. It is needed for
proper handling of offline files as performing regular I/O on offline file will
block smbd.

Signed-off-by: Alexander Bokovoy <ab@samba.org>