Enhance vfs.ino64_trunc_error sysctl.

Provide a new mode "2" which returns a special overflow indicator in
the non-representable field instead of the silent truncation (mode
"0") or EOVERFLOW (mode "1").

In particular, the typical use of st_ino to detect hard links with
mode "2" reports false positives, which might be more suitable for
some uses.

Discussed with: bde
Sponsored by: The FreeBSD Foundation