Refactor tcp_get_srtt()

[jtl]

Summary

After rGf5abdb03119a, it looks like there is an opportunity to refactor tcp_get_srtt() into its two component operations: unit conversion and shifting.

This should not cause any functional change.

Note for the future: I question whether it is a good idea for the long-term to manage the complexity of maintaining alternate unit storage in the TCB. If there is not already a plan for unifying the unit used for these values, I think we should consider making one.

[tuexen]

There is the plan to move the base stack away from ticks and use usecs everywhere. But we wanted not to do this before stable/14 is branched.