This takes Warners suggested approach to making it so that
platforms that for whatever reason cannot include the RATELIMIT option
can still work with rack. It adds two dummy functions that rack will
call and find out that the highest hw supported b/w is 0 (which
kinda makes sense and rack is already prepared to handle).

Reviewed by: Michael Tuexen, Warner Losh
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30163

(cherry picked from commit 5a4333a5378f7afe4f8cab293a987865ae0c32c4)