Minimum wrapfs rename

wrapfs renamed to idmapfs, with minimum changes.

Signed-off-by: Alin Dobre <alinmd@gmail.com>
Mapping for multiple operations

UID-only mapping is now done properly for stat and read access
functionalities.

Signed-off-by: Alin Dobre <alinmd@gmail.com>
Mapping read/write with fsuid

The fsuid mapping should replaces modifying the inode directly for
read/write operations.

Signed-off-by: Alin Dobre <alinmd@gmail.com>