Fix -delete on FreeBSD NFS mounts

[tavianator]

Doing unlink() during readdir() on FreeBSD makes it skip some directory entries: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=57696

Easily reproducible with

$ mkdir foo && touch foo/{1..256}
$ bfs foo -delete
bfs: error: foo: Directory not empty.
$ bfs foo -delete
$

However, find, rm, etc. seem to work around it. truss indicates that they fully read the directory before starting to remove files. That means we have some workarounds:

$ mkdir foo && touch foo/{1..256}
$ ./bfs -S dfs foo -delete
$ mkdir foo && touch foo/{1..256}
$ ./bfs -s foo -delete
$

But it would be nice if plain bfs worked too.