tcp: make CUBIC the default congestion control mechanism.

This changes the default TCP Congestion Control (CC) to CUBIC.
For small, transactional exchanges (e.g. web objects <15kB), this
will not have a material effect. However, for long duration data
transfers, CUBIC allocates a slightly higher fraction of the
available bandwidth, when competing against NewReno CC.

Reviewed By: tuexen, mav, transport, guest-ccui, emaste
Relnotes: Yes
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D36537