tree-wide: Update to new libglnx fd APIs

This ends up a lot better IMO.  This commit is *mostly* just
`s/glnx_close_fd/glnx_autofd`, but there's also a number of hunks like:

```
-  if (self->sysroot_fd != -1)
-    {
-      (void) close (self->sysroot_fd);
-      self->sysroot_fd = -1;
-    }
+  glnx_close_fd (&self->sysroot_fd);
```

Update submodule: libglnx

Closes: #1259
Approved by: jlebon

src/libostree/ostree-repo-finder-mount.c

@@ -328,9 +328,9 @@ ostree_repo_finder_mount_resolve_async (OstreeRepoFinder                  *finde
       g_autofree gchar *mount_name = NULL;
       g_autoptr(GFile) mount_root = NULL;
       g_autofree gchar *mount_root_path = NULL;
-      glnx_fd_close int mount_root_dfd = -1;
+      glnx_autofd int mount_root_dfd = -1;
       struct stat mount_root_stbuf;
-      glnx_fd_close int repos_dfd = -1;
+      glnx_autofd int repos_dfd = -1;
       gsize i;
       g_autoptr(GHashTable) repo_to_refs = NULL;  /* (element-type UriAndKeyring GHashTable) */
       GHashTable *supported_ref_to_checksum;  /* (element-type OstreeCollectionRef utf8) */