recursedir: introduce new RECURSE_DIR_UNLINK callback return value

recursedir: introduce new RECURSE_DIR_UNLINK callback return value

This introduces RECURSE_DIR_UNLINK + RECURSE_DIR_UNLINK_GRACEFUL as new
return values for recurse_dir() callbacks. If either is returned the
inode currently processed will be removed and iteration continues with
the next item.

This is useful to have in the recurse_dir() mechanism itself (rather
than implement in the callback itself), due to ordering: we want to pin
the inodes via an fd while calling the callbacks, but we have to close
it before removal of the inodes. By moving this into the recurse_dir()
infra, we can implement this easily.