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-gpg-verifier.c

@@ -177,7 +177,7 @@ _ostree_gpg_verifier_check_signature (OstreeGpgVerifier  *self,
       for (guint i = 0; i < self->key_ascii_files->len; i++)
         {
           const char *path = self->key_ascii_files->pdata[i];
-          glnx_fd_close int fd = -1;
+          glnx_autofd int fd = -1;
           g_auto(gpgme_data_t) kdata = NULL;
 
           if (!glnx_openat_rdonly (AT_FDCWD, path, TRUE, &fd, error))
@@ -355,7 +355,7 @@ _ostree_gpg_verifier_add_keyring_dir_at (OstreeGpgVerifier   *self,
       if (g_str_equal (name, "secring.gpg"))
         continue;
 
-      glnx_fd_close int fd = -1;
+      glnx_autofd int fd = -1;
       if (!glnx_openat_rdonly (dfd_iter.fd, dent->d_name, TRUE, &fd, error))
         return FALSE;