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>

src/pxar/encoder.rs

@@ -287,7 +287,7 @@ impl<'a, W: Write, C: BackupCatalogWriter> Encoder<'a, W, C> {
             Err(err) => bail!("read_xattrs failed for {:?} - {}", self.full_path(), err),
         };
 
-        for name in xattr_names.split(|c| *c == b'\0') {
+        for name in &xattr_names {
             // Only extract the relevant extended attributes
             if !xattr::is_valid_xattr_name(&name) {
                 continue;
@@ -307,7 +307,7 @@ impl<'a, W: Write, C: BackupCatalogWriter> Encoder<'a, W, C> {
                 }
             } else if self.has_features(flags::WITH_XATTRS) {
                 xattrs.push(PxarXAttr {
-                    name: name.to_vec(),
+                    name: name.to_bytes().to_vec(),
                     value,
                 });
             }