fix #3393 (again): pxar/create: try to read xattrs/fcaps/acls by default

fix #3393 (again): pxar/create: try to read xattrs/fcaps/acls by default

we have a static list of filesystems and their capabilities regarding
file attributes and fs features (e.g. sockets/fifos/etc) which also
includes xattrs,acls and fcaps

if we did not know a filesystem by its magic number (for example cephfs),
we did not even attempt to read xattrs, etc.

this patch adds those flags by default to unknown filesystems, and
removes them when we encounter EOPNOTSUPP (to remove the number
of syscalls)

with this, we should be able to catch xattrs/acls/fcaps on all
(unknown) fs types that support them

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>