This small change adopts the excellent suggestion for using named
structures in the add of a new tcp-stack that came in late to me
via email after the last commit. It also makes it so that a new
stack may optionally get a callback during a retransmit
timeout. This allows the new stack to clear specific state (think
sack scoreboards or other such structures).

Sponsored by: Netflix Inc.
Differential Revision: http://reviews.freebsd.org/D6303