This commit expands tcp_ratelimit to be able to handle cards

This commit expands tcp_ratelimit to be able to handle cards
like the mlx-c5 and c6 that require a "setup" routine before
the tcp_ratelimit code can declare and use a rate. I add the
setup routine to if_var as well as fix tcp_ratelimit to call it.
I also revisit the rates so that in the case of a mlx card
of type c5/6 we will use about 100 rates concentrated in the range
where the most gain can be had (1-200Mbps). Note that I have
tested these on a c5 and they work and perform well. In fact
in an unloaded system they pace right to the correct rate (great
job mlx!). There will be a further commit here from Hans that
will add the respective changes to the mlx driver to support this
work (which I was testing with).

Sponsored by:	Netflix Inc.
Differential Revision:	ttps://reviews.freebsd.org/D23647