xattr: api cleanup

xattr: api cleanup

Make `flistxattr()` return a `ListXAttr` helper which
provides an iterator over `&CStr`.

This exposes the property that xattr names are a
zero-terminated string without simply being an opaque
"byte vector". Using &[u8] as a type here is too lax.

Also let `fgetxattr` take a `CStr`. While this may be a
burden on the caller, we usually already have
zero-terminated strings on the call site. Currently we only
use this method coming from `flistxattr` after all.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>