Improve SACK support code for RFC6675 and PRR

Adding proper accounting of sacked_bytes and (per-ACK)
delivered data to the SACK scoreboard. This will
allow more aspects of RFC6675 to be implemented as well
as Proportional Rate Reduction (RFC6937).

Prior to this change, the pipe calculation controlled with
net.inet.tcp.rfc6675_pipe was also susceptible to incorrect
results when more than 3 (or 4) holes in the sequence space
were present, which can no longer all fit into a single
ACK's SACK option.

Reviewed by: kbowling, rgrimes (mentor)
Approved by: rgrimes (mentor, blanket)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D18624