s3: Add support for storing dos attributes as st_flags in the stat struct.

s3: Add support for storing dos attributes as st_flags in the stat struct.

Some filesystems have support for storing dos attributes directly in
the inode's st_flags and accessing them through the stat struct.  This
patch:

- Adds a configure check to see if the special flags are available.
- Implements getting and setting dos attributes in the stat struct and
  inode, respectively.

This will not change the existing functionality of any system that
doesn't have the special flags available.