gc: remove .bad files on garbage collect

gc: remove .bad files on garbage collect

The iterator of get_chunk_iterator is extended with a third parameter
indicating whether the current file is a chunk (false) or a .bad file
(true).

Count their sizes to the total of removed bytes, since it also frees
disk space.

.bad files are only deleted if the corresponding chunk exists, i.e. has
been rewritten. Otherwise we might delete data only marked bad because
of transient errors.

While at it, also clean up and use nix::unistd::unlinkat instead of
unsafe libc calls.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>