tcp: various improvements and fixes to PRR

• Ensure cwnd doesn't shrink to zero with PRR
• use accurate rfc6675_pipe when enabled
• Address two incorrect calculations and enhance readability
• address second instance of cwnd potentially becoming zero
• fix sublte bug due to implicit int to uint typecase in max()
• fix bug due to typo in hand-coded CEILING() function by using howmany() macro
• use int instead of long, and add a missing long typecast
• replace if conditionals with easier to read imax/imin (as in pseudocode)
• Avoid accounting left-edge twice in partial ACK.
• Include new data sent in PRR calculation
• Improve PRR initial transmission timing
• Fix prr_out when pipe < ssthresh

Reviewed By: transport, kbowling, tuexen
Reported by: Liang Tian
Approved by: re (gjb)
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D28780
Differential Revision: https://reviews.freebsd.org/D28816
Differential Revision: https://reviews.freebsd.org/D28813
Differential Revision: https://reviews.freebsd.org/D28819
Differential Revision: https://reviews.freebsd.org/D28941
Differential Revision: https://reviews.freebsd.org/D28953
Differential Revision: https://reviews.freebsd.org/D28998

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