fts: refactor to use fd-relative operations internally

Replace all _open() calls with _openat() in __fts_open(), fts_read(),
and fts_children().

Add fts_dirfd to FTSENT. Callers can use
openat(ent->fts_dirfd, ent->fts_name, ...) to access files
safely without relying on fts_accpath, which enables:

1. Capsicum capability mode where path-based operations fail
2. Security-sensitive programs that avoid TOCTOU races

Replace statfs(ent->fts_path) with _fstatfs(ent->fts_dirfd) in
fts_ufslinks() when fts_dirfd is valid, falling back to statfs() for
root-level entries where fts_dirfd is -1

This is a preparatory change for fts_openat() which will allow
callers to provide a pre-opened directory fd, enabling fts(3)
traversal inside Capsicum capability mode.

Sponsored by: Google LLC (GSoC 2026)
Reviewed by: asomers, jillest
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/2278