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.h

@@ -537,12 +537,14 @@ typedef OstreeRepoCommitFilterResult (*OstreeRepoCommitFilter) (OstreeRepo    *r
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_SKIP_XATTRS: Do not process extended attributes
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES: Generate size information.
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CANONICAL_PERMISSIONS: Canonicalize permissions for bare-user-only mode.
+ * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_ERROR_ON_UNLABELED: Emit an error if configured SELinux policy does not provide a label
  */
 typedef enum {
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_NONE = 0,
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_SKIP_XATTRS = (1 << 0),
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES = (1 << 1),
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CANONICAL_PERMISSIONS = (1 << 2),
+  OSTREE_REPO_COMMIT_MODIFIER_FLAGS_ERROR_ON_UNLABELED = (1 << 3),
 } OstreeRepoCommitModifierFlags;
 
 /**