TCP newreno: improve after_idle ssthresh

Adjust ssthresh in after_idle to the maximum of
the prior ssthresh, or 3/4 of the prior cwnd. See
RFC2861 section 2 for an in depth explanation for
the rationale around this.

As newreno is the default "fall-through" reaction,
most tcp variants will benefit from this.

Reviewed by: tuexen
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D22438