Initial fs-verity support

Using fs-verity is natural for OSTree because it's file-based,
as opposed to block based (like dm-verity).  This only covers
files - not symlinks or directories.  And we clearly need to
have integrity for the deployment directories at least.

Also, what we likely need is an API that supports signing files
as they're committed.

So making this truly secure would need a lot more work.  Nevertheless,
I think it's time to start experimenting with it.  Among other things,
it does *finally* add an API that makes files immutable, which will
help against some accidental damage.

This is basic enablement work that is being driven by
Fedora CoreOS; see also https://github.com/coreos/coreos-assembler/pull/876

src/libostree/ostree-repo.c

@@ -31,6 +31,7 @@
 #include "libglnx.h"
 #include "otutil.h"
 #include <glnx-console.h>
+#include <linux/magic.h>
 
 #include "ostree-core-private.h"
 #include "ostree-sysroot-private.h"
@@ -47,6 +48,7 @@
 #include <glib/gstdio.h>
 #include <sys/file.h>
 #include <sys/statvfs.h>
+#include <sys/statfs.h>
 
 #define REPO_LOCK_DISABLED (-2)
 #define REPO_LOCK_BLOCKING (-1)
@@ -3033,6 +3035,34 @@ reload_core_config (OstreeRepo          *self,
       }
   }
 
+  /* Currently experimental */
+  static const char fsverity_key[] = "ex-fsverity";
+  self->fs_verity_wanted = _OSTREE_FEATURE_NO;
+#ifdef HAVE_LINUX_FSVERITY_H
+  self->fs_verity_supported = _OSTREE_FEATURE_MAYBE;
+#else
+  self->fs_verity_supported = _OSTREE_FEATURE_NO;
+#endif
+  gboolean fsverity_required = FALSE;
+  if (!ot_keyfile_get_boolean_with_default (self->config, fsverity_key, "required",
+                                            FALSE, &fsverity_required, error))
+    return FALSE;
+  if (fsverity_required)
+    {
+      self->fs_verity_wanted = _OSTREE_FEATURE_YES;
+      if (self->fs_verity_supported == _OSTREE_FEATURE_NO)
+        return glnx_throw (error, "fsverity required, but libostree compiled without support");
+    }
+  else
+    {  
+      gboolean fsverity_opportunistic = FALSE;
+      if (!ot_keyfile_get_boolean_with_default (self->config, fsverity_key, "opportunistic",
+                                                FALSE, &fsverity_opportunistic, error))
+        return FALSE;
+      if (fsverity_opportunistic)
+        self->fs_verity_wanted = _OSTREE_FEATURE_MAYBE;
+    }
+  
   {
     g_clear_pointer (&self->collection_id, g_free);
     if (!ot_keyfile_get_value_with_default (self->config, "core", "collection-id",