ostree-repo-libarchive.c: major refactor

- Make hardlink handling more generic. The previous strategy worked for
  tar archives, but not for cpio. It now works for both.
- Add support for SEL labeling (through the OstreeRepoCommitModifier)
- Add support for xattr_callback (through the OstreeRepoCommitModifier)
- Add support for filter (through the OstreeRepoCommitModifier)
- Add a use_ostree_convention option

Closes: #275
Approved by: cgwalters

src/libostree/ostree-repo.h

@@ -582,7 +582,9 @@ gboolean      ostree_repo_write_archive_to_mtree (OstreeRepo                   *
 typedef struct {
   guint ignore_unsupported_content : 1;
   guint autocreate_parents : 1;
-  guint reserved : 30;
+  guint use_ostree_convention : 1;
+  guint callback_with_entry_pathname : 1;
+  guint reserved : 28;
 
   guint unused_uint[8];
   gpointer unused_ptrs[8];