fts: refactor to use fd-relative operations internally

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

Add fts_dirfd to struct _ftsent, set to the file descriptor of the
parent directory. Callers can use openat(ent->fts_dirfd, ent->fts_name,
...) to access files safely without relying on fts_accpath, which
enables programs in capability mode to open the files described by
_ftsent.

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.

Mirror all fts_open() changes to fts_open_b().

As a result of expanding _ftsend, publish new ELF symbol versions for
fts_openat and related functions.

Sponsored by:	Google LLC (GSoC 2026)
Reviewed by:	asomers
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2303

include/fts.h

@@ -92,6 +92,8 @@ struct _ftsent {
 	char *fts_path;			/* root path */
 	int fts_errno;			/* errno for this node */
 	int fts_symfd;			/* fd for symlink */
+	int fts_dirfd;                  /* fd for parent directory */
+	int __fts_reserved[3];          /* reserved for future use */
 	__size_t fts_pathlen;		/* strlen(fts_path) */
 	__size_t fts_namelen;		/* strlen(fts_name) */