Improve DSACK support:

MFC This is the second in a number of patches needed to
get BBRv1 into the tree. This fixes the DSACK bug but
is also needed by BBR. We have yet to go two more
one will be for the pacing code (tcp_ratelimit.c) and
the second will be for the new updated LRO code that
allows a transport to know the arrival times of packets
and (tcp_lro.c). After that we should finally be able
Improve DSACK support.

MFC r349987 from rrs@:

This is the second in a number of patches needed to
get BBRv1 into the tree. This fixes the DSACK bug but
is also needed by BBR. We have yet to go two more
one will be for the pacing code (tcp_ratelimit.c) and
the second will be for the new updated LRO code that
allows a transport to know the arrival times of packets
and (tcp_lro.c). After that we should finally be able
to get BBRv1 into head.

This required manual tweaking to address merge conflicts.

MFC r351725:

This patch improves the DSACK handling to conform with RFC 2883.
The lowest SACK block is used when multiple Blocks would be elegible as
DSACK blocks ACK blocks get reordered - while maintaining the ordering of
SACK blocks not relevant in the DSACK context is maintained.

This required manual tweaking to address merge conflicts.

MFC r351801:

Fix the SACK block generation in the base TCP stack by bringing it in
sync with the RACK stack.