Add flag to make SELinux label failure fatal, add hack for /proc

I was working on `rpm-ostree livefs` which does some ostree-based
filesystem diffs, and noticed that we were ending up with `/proc`
not being labeled in our base trees.

Reading the selinux-policy source, indeed we have:

```
/proc			-d	<<none>>
/proc/.*			<<none>>
```

This dates pretty far back.  We really don't want unlabeled
content in ostree.  In this case it's mostly OK since the kernel
will assign a label, but again *everything* should be labeled via
OSTree so that it's all consistent, which will fix `ostree diff`.

Notably, `/proc` is the *only* file path that isn't covered when composing a
Fedora Atomic Host. So I added a hack here to hardcode it (although I'm a bit
uncertain about whether it should really be `proc_t` on disk before systemd
mounts or not).

Out of conservatism, I made this a flag, so if we hit issues down the line, we
could easily change rpm-ostree to stumble on as it did before.

Closes: #768
Approved by: jlebon

src/libostree/ostree-repo-commit.c

@@ -2309,7 +2309,11 @@ get_modified_xattrs (OstreeRepo                       *self,
                                       &label, cancellable, error))
         return FALSE;
 
-      if (label)
+      if (!label && (modifier->flags & OSTREE_REPO_COMMIT_MODIFIER_FLAGS_ERROR_ON_UNLABELED) > 0)
+        {
+          return glnx_throw (error, "Failed to look up SELinux label for '%s'", relpath);
+        }
+      else if (label)
         {
           g_autoptr(GVariantBuilder) builder = NULL;