fts: Move private flags away from public ones.

Renumber the private flags so there is a sizeable gap between them and
the public flags, making it easier to add public flags in the future.
These private flags are only ever set or read by FTS itself, so there
is no compatibility issue.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49711

(cherry picked from commit 80e06d621ac545ac0a69d4e95e7392be38013a78)