tcp: Add and update cubic module variable names

Prepare the cubic congestion control module to better align with
the specifications in RFC8312bis.

Rename a few cubic state variables to the variable names found in
the RFC8312bis specification. This makes the code more understandable
for someone reading the RFC and the code. It also makes the variable
naming convention more uniform. Add some variables needed subsequently.

No functional change.

Submitted By: Bhaskar Pardeshi, VMware Inc.
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D40436