Add TCP feature Proportional Rate Reduction (PRR) - RFC6937

PRR improves loss recovery and avoids RTOs in a wide range
of scenarios (ACK thinning) over regular SACK loss recovery.

PRR is disabled by default, enable by net.inet.tcp.do_prr = 1.
Performance may be impeded by token bucket rate policers at
the bottleneck, where net.inet.tcp.do_prr_conservate = 1
should be enabled in addition.

Submitted by: Aris Angelogiannopoulos
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D18892