On FreeBSD + zfs, stat call seem to return garbage in RDev for regular
files & folders. The value returned is large enough not to fit into
`Devmajor` & `Devminor` fields of the tar header. Fortunately, these
fields are required just for special devices.
This change
* adds a check into `setHeaderForSpecialDevice` that the
input header represents a special device. If it's not the case, we
don't set the Devmajor & Devminor fields.
* fixes the nil check on `getxattr`: it never returns nils, but rather
an empty slice instead
Signed-off-by: Artem Khramov <akhramov@pm.me>