Deduplicate and fix up our use of mmap()

Buried in this large patch is a logical fix:

```
-  if (!map)
-    return glnx_throw_errno_prefix (error, "mmap");
+  if (map == (void*)-1)
+    return glnx_null_throw_errno_prefix (error, "mmap");
```

Which would have helped me debug another patch I was working
on.  But it turns out that actually correctly checking for
errors from `mmap()` triggers lots of other bugs - basically
because we sometimes handle zero-length variants (in detached
metadata).  When we start actually returning errors due to
this, things break.  (It wasn't a problem in practice before
because most things looked at the zero size, not the data).

Anyways there's a bigger picture issue here - a while ago
we made a fix to only use `mmap()` for reading metadata from disk
only if it was large enough (i.e. `>16k`).  But that didn't
help various other paths in the pull code and others that were
directly doing the `mmap()`.

Fix this by having a proper low level fs helper that does "read all data from
fd+offset into GBytes", which handles the size check. Then the `GVariant` bits
are just a clean layer on top of this. (At the small cost of an additional
allocation)

Side note: I had to remind myself, but the reason we can't just use
`GMappedFile` here is it doesn't support passing an offset into `mmap()`.

Closes: #1251
Approved by: jlebon

src/libostree/ostree-repo-commit.c

@@ -2212,26 +2212,29 @@ ostree_repo_read_commit_detached_metadata (OstreeRepo      *self,
   char buf[_OSTREE_LOOSE_PATH_MAX];
   _ostree_loose_path (buf, checksum, OSTREE_OBJECT_TYPE_COMMIT_META, self->mode);
 
-  g_autoptr(GVariant) ret_metadata = NULL;
-  if (self->commit_stagedir.initialized &&
-      !ot_util_variant_map_at (self->commit_stagedir.fd, buf,
-                               G_VARIANT_TYPE ("a{sv}"),
-                               OT_VARIANT_MAP_ALLOW_NOENT | OT_VARIANT_MAP_TRUSTED, &ret_metadata, error))
-    return glnx_prefix_error (error, "Unable to read existing detached metadata");
+  if (self->commit_stagedir.initialized)
+    {
+      glnx_fd_close int fd = -1;
+      if (!ot_openat_ignore_enoent (self->commit_stagedir.fd, buf, &fd, error))
+        return FALSE;
+      if (fd != -1)
+        return ot_variant_read_fd (fd, 0, G_VARIANT_TYPE ("a{sv}"), TRUE,
+                                   out_metadata, error);
+    }
 
-  if (ret_metadata == NULL &&
-      !ot_util_variant_map_at (self->objects_dir_fd, buf,
-                               G_VARIANT_TYPE ("a{sv}"),
-                               OT_VARIANT_MAP_ALLOW_NOENT | OT_VARIANT_MAP_TRUSTED, &ret_metadata, error))
-    return glnx_prefix_error (error, "Unable to read existing detached metadata");
+  glnx_fd_close int fd = -1;
+  if (!ot_openat_ignore_enoent (self->objects_dir_fd, buf, &fd, error))
+    return FALSE;
+  if (fd != -1)
+    return ot_variant_read_fd (fd, 0, G_VARIANT_TYPE ("a{sv}"), TRUE,
+                               out_metadata, error);
 
-  if (ret_metadata == NULL && self->parent_repo)
+  if (self->parent_repo)
     return ostree_repo_read_commit_detached_metadata (self->parent_repo,
-                                                      checksum,
-                                                      out_metadata,
-                                                      cancellable,
-                                                      error);
-  ot_transfer_out_value (out_metadata, &ret_metadata);
+                                                      checksum, out_metadata,
+                                                      cancellable, error);
+  /* Nothing found */
+  *out_metadata = NULL;
   return TRUE;
 }