Log Message:
Update to 2.7.

* Build improvements, use pkgconfig.
* Don't add rpc2's 100ms delayed ack delay in SFTP roundtrip estimate.
* Reduce the number of packets SFTP drops when we receive duplicates.
* Retransmit more worried packets (which 'should' have been acked).
* Only retransmit on timeouts, have the active side of the connection be
  mostly timeout driven to avoid duplicate transfers.