support selinux relabling APIs

Split from https://github.com/rhinstaller/anaconda/pull/7236

Basically we have ~three types of data:

1) stuff in the ostree/composefs repo that should be immutable
2) `/etc` and `/var`
3) Other mounted partitions

I think we should offer APIs to perform relabeling of the first two. For the first, we already have `bootc internals fsck` that should handle stuff like this. I think that doesn't need to be run from the initramfs.

For the second, probably the simplest thing here is `/etc/.autorelabel` and `/var/.autorelabel` or so? I think semantically, these would not traverse mountpoints (see below).

For other mounted partitions, I think SELinux userspace in the initramfs should probably support something for this.

Start by reviewing the split from pull request 7236 and the existing `bootc internals fsck` entry point. Compare the proposed handling for the ostree/composefs repository, `/etc` and `/var`, and other mounted partitions, including SELinux userspace in the initramfs. Done requires an agreed API scope and implementation plan for relabeling each applicable data category.

Written by the indexing model from the issue text.