Traversal: skip folder if size < minsize

[aowenson-imm]

Implements #662.

/* skip folder if size < minsize
* In some filesystems e.g. Ceph, folder size = 
* recursive sum of all contents. */

Massive performance improvement for me (Ceph filesystem, skipping folders < 1 GB), been using occasionally for several weeks now. If your filesystem does not do recursive folder sizing e.g. ext4, then no effect. Not sure what other filesystems implement this other than Ceph.

[aowenson-imm]

Test fail:

        *_, footer = run_rmlint('--size 1024')
>       assert footer['duplicates'] == 4
E       assert 0 == 4

Cause is my folder-skip logic on "non-recursive-size" filesystems e.g. ext4 is skipping all folders (because size < 1KB).

Two possible solutions:

1. auto-detect if filesystem implements recursive-size for folders (does folder size exceed its metadata size)
2. new CLI argument to request applying size-skipping to folders

Which do you prefer?

[vassilit]

rmlint already do detect filesystems for some options (support of ioctls as FICLONERANGE, etc).

However, I think that the st_size value of CephFS with the rbytes feature enabled is non-standard, so I would prefer a new CLI long option.