Reserve u2f group for FIDO/U2F key support (SSH, etc.)

We have FIDO/U2F support in the base system now, so reserve a group ID
for it (maching the security/u2f-devd port).

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34341

(cherry picked from commit d0f1f38202c4176af277185a3b973098bb03fe6d)