nsh clears input temporarily

While using the mission planner terminal screen's NSH, the user's input is temporarily cleared after the user presses enter. It appears again once the command's results are returned by ardupilot.

Perhaps related to this, i seems if there's output coming from ardupilot, it upsets the processing of the input from the user. With AC3.3-rc8 it seems the "Overtime in task 19" is appearing fairly often (something that I can look into) but i seems that this message arriving from ardupilot upsets the processing of the user's input meaning the user must be very quick to enter the command they want and press enter before another string arrives.

Reproduce the issue in the Mission Planner terminal screen's NSH while commands and ArduPilot output arrive together, using the AC3.3-rc8 behavior described. Start by tracing the terminal input and output handling; done means entered text no longer disappears or gets disrupted when command results or messages such as "Overtime in task 19" arrive.

Written by the indexing model from the issue text.