prepare-root: boot is kept writable

[cgwalters]

If we don't have a separate /boot today, then the prepare root code makes a bind mount to the physical path.

In the case of sysroot.readonly=true, this is done before we set up the readonly state, leaving /boot writable which I think is not expected.

Separately but definitely related to this we have this code in the sysroot logic:

  if (!remount_writable ("/boot", &did_remount_boot, error))
    return FALSE;

This checks for ST_RDONLY and tries to remount if it finds that, but that's broken - we need to check if it's actually a mountpoint instead.

These bugs seem to currently cancel out, but I saw at least one person reporting

$ bootc status
ERROR Status: Initializing storage: Acquiring sysroot: Remounting /boot read-write: Invalid argument

which is almost certainly hitting the latter case somehow.

Fixes:

• Detect mountpoint in sysroot code using statx (ref https://github.com/coreos/cap-std-ext/blob/8737440db94fea9b17ba1e99c078103c1f3b3ce9/src/dirext.rs#L311 )
• Consider changing /boot to be readonly if sysroot.readonly - but we need to carefully consider compatibility