tcp_rack: Add a manual page

• add a manual page for tcp_rack(4)
• link it in the tcp(4) and tcp_bbr(4) man pages
• hook it up to build in the Makefile

Reviewed by: pauamma
Differential Revision: https://reviews.freebsd.org/D37209

(cherry picked from commit bdbc05bea995d73465def1ab31bb3b2b37b37a93)