tcp/sack: Force out more segments allowed by "pipe" during fast recovery

tcp/sack: Force out more segments allowed by "pipe" during fast recovery

If some segments are cumulatively acked or SACKed, and HighRxt equals
snd_una, one segment (new or retransmit) will be forced out even if cwnd
and pipe don't allow it.  When large amount of segments are lost, i.e.
computed pipe could be large, this avoids unnecessary retransmit timeout
and could perform as good as NewReno.

Sysctl node net.inet.tcp.force_sackrxt could be tuned to burst out several
retransmits, default is 1 (should be good enough).  Set this sysctl to 0,
SACK based fast recovery will obey the computed pipe.

Several unnecessary retransmit timeout graph as described above:
http://leaf.dragonflybsd.org/~sephe/no_force_sack_rexmt2_15.xpl (starts @15s)
http://leaf.dragonflybsd.org/~sephe/no_force_sack_rexmt_54.xpl (starts @54s)