Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Import the ERTT (Enhanced Round Trip Time) Khelp module. ERTT uses the
Khelp/Hhook KPIs to hook into the TCP stack and maintain a per-connection, low
noise estimate of the instantaneous RTT. ERTT's implementation is robust even in
the face of delayed acknowledgements and/or TSO being in use for a connection.

A high quality, low noise RTT estimate is a requirement for applications such as
delay-based congestion control, for which we will be importing some algorithm
implementations shortly.

In collaboration with: David Hayes <dahayes at swin edu au> and
Grenville Armitage <garmitage at swin edu au>
Sponsored by: FreeBSD Foundation
Reviewed by: bz and others along the way
MFC after: 3 months