tools/logrotate: fix compression logic

tools/logrotate: fix compression logic

we never actually compressed any files, since we only looked at
the extension:
* if it was 'zst' (which was always true for newly rotated files), we
  would not compress it
* even if it was not 'zst', we compressed it inplace, never adding '.zst'
  (possibly compressing them multiple times as zstd)

now we add new rotated files simply as '.X' and add a 'target' to the
compress fn, where we rename it to (but now we have to unlink the source
path)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>