Rack cannot be loaded without cc_newreno compiled into the kernel.

Right now rack will fail to load due to its hack in accessing symbol names
in cc_newreno. This was fine when newreno was always compiled into the
kernel but now ... not so much. Instead lets fix up rack to use the socket
option queries to get the information it wants and set the parameters. We
also fix the CC parameter so they are always settable.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D37622