chcon: walk directories with DirFd instead of fts

Reuse uucore::safe_traversal, the same traversal chown and chgrp already
use, in place of fts(3). Directories are still labelled post-order, so a
context that forbids reading one is applied only once its contents are
done.

Cycles are now caught with a set of the directories open on the current
path rather than FTS_DC, and the stale-stat retry goes away: entries are
stat'ed when they are visited rather than when the walk is opened.

chcon was the only fts-sys user in the workspace, so the dependency goes
with it.

Author: sylvestre

Cargo.lock

@@ -23,7 +23,6 @@ uucore = { workspace = true, features = ["entries", "fs", "perms"] }
 selinux = { workspace = true }
 thiserror = { workspace = true }
 libc = { workspace = true }
-fts-sys = { workspace = true }
 rustix = { workspace = true, features = ["fs"] }
 fluent = { workspace = true }