tcp: better congestion control defaults

Define CC_NEWRENO in all the appropriate DEFAULTS and std.* config
files. It's the default congestion control algorithm. Add code to cc.c
so that CC_DEFAULT is "newreno" if it's not overriden in the config
file.

Sponsored by: Netflix
Fixes: b8d60729deef ("tcp: Congestion control cleanup.")
Revired by: manu, hselasky, jhb, glebius, tuexen
Differential Revision: https://reviews.freebsd.org/D32964