Seems that you need to have read everything you want before you
call waitpid, otherwise the pending data seems to be snipped and
prevented from being read.
closes: https://github.com/wez/wezterm/issues/187
It's too fiddly to setup in practice, and literally no one has
expressed an interest in using it.
Removing it simplifies some upcoming work.
Closes: https://github.com/wez/wezterm/issues/35