mountpoint-util: rework name_to_handle_at() unique mount id handling

mountpoint-util: rework name_to_handle_at() unique mount id handling

name_to_handle_at_try_unique_mntid_fid() in its current form is
ill-designed for various reasons:

* AT_HANDLE_FID requires file system support, while unique mount id
  is a VFS concept hence is always available if supported. Hence
  the fallback for AT_HANDLE_MNT_ID_UNIQUE should be independent
  of fid.
* The request for AT_HANDLE_MNT_ID_UNIQUE can be identified via
  specifying ret_unique_mnt_id, no need for opening up the control
  to caller (and currently the function simply doesn't handle
  mismatch between ret params and flags).
* The caller cannot realistically differentiate whether the returned
  mount id is actually unique.
* The path_get_unique_mnt_id() fallback did not handle AT_SYMLINK_FOLLOW.

Let's instead move the statx() fallback into name_to_handle_at_loop()
directly, and revamp interaction of ret_mnt_id/ret_unique_mnt_id:
if both are set, it indicates that the caller can handle both, hence
set what we have and return 0/1 for whether we managed to acquire
the unique one.

The !ret_handle && ret_mnt_id logic is removed. Let's not rely on
undocumented bizaare behavior and it's unused anyways.
path_get_mnt_id_at() exists for a reason...