Port to GLnxTmpfile

There's lots of mechanically replacing `OtTmpFile` with `GLnxTmpfile`;
the biggest changes are in the commit path.  Symlink commits are now
very clearly separated from regular files.  Symlinks are `OtCleanupUnlinkat`,
and regular files are `GLnxTmpfile`.

The commit codepath separates those as `_ostree_repo_commit_path_final()` and
`_ostree_repo_commit_tmpf_final()`. A nice aspect of all of this is that they
both *consume* the temporary on success. This avoids an extra spurious
`unlink()` call.

One of the biggest bits of code motion is in `commit_loose_regfile_object()`,
which no longer needs to care about symlinks. For the most parth though it's
just removing conditionals.

Update submodule: libglnx

Closes: #958
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -296,20 +296,26 @@ _ostree_repo_verify_commit_internal (OstreeRepo    *self,
                                      GError       **error);
 
 gboolean
-_ostree_repo_commit_loose_final (OstreeRepo        *self,
-                                 const char        *checksum,
-                                 OstreeObjectType   objtype,
-                                 int                temp_dfd,
-                                 int                fd,
-                                 const char        *temp_filename,
-                                 GCancellable      *cancellable,
-                                 GError           **error);
+_ostree_repo_commit_tmpf_final (OstreeRepo        *self,
+                                const char        *checksum,
+                                OstreeObjectType   objtype,
+                                GLnxTmpfile       *tmpf,
+                                GCancellable      *cancellable,
+                                GError           **error);
+
+gboolean
+_ostree_repo_commit_path_final (OstreeRepo        *self,
+                                const char        *checksum,
+                                OstreeObjectType   objtype,
+                                OtCleanupUnlinkat *tmp_path,
+                                GCancellable      *cancellable,
+                                GError           **error);
 
 gboolean
 _ostree_repo_open_content_bare (OstreeRepo          *self,
                                 const char          *checksum,
                                 guint64              content_len,
-                                OtTmpfile           *out_tmpf,
+                                GLnxTmpfile         *out_tmpf,
                                 gboolean            *out_have_object,
                                 GCancellable        *cancellable,
                                 GError             **error);
@@ -317,7 +323,7 @@ _ostree_repo_open_content_bare (OstreeRepo          *self,
 gboolean
 _ostree_repo_commit_trusted_content_bare (OstreeRepo          *self,
                                           const char          *checksum,
-                                          OtTmpfile           *tmpf,
+                                          GLnxTmpfile         *tmpf,
                                           guint32              uid,
                                           guint32              gid,
                                           guint32              mode,