Impose a maximum length on lines read from child processes

[jwodder]

This will be necessary in order to avoid massive memory usage due to bad commands, but what exactly should happen when a line exceeds the maximum length? Possibilities:

• Error
• Discard everything after the max length cutoff point
• Output everything up to the max length with a newline added
  • Try to figure out the control sequences to add further output to the end of the previous line