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-bootloader-grub2.c

@@ -416,7 +416,7 @@ _ostree_bootloader_grub2_write_config (OstreeBootloader      *bootloader,
     }
 
   /* Now let's fdatasync() for the new file */
-  { glnx_fd_close int new_config_fd = -1;
+  { glnx_autofd int new_config_fd = -1;
     if (!glnx_openat_rdonly (AT_FDCWD, gs_file_get_path_cached (new_config_path), TRUE, &new_config_fd, error))
       return FALSE;