Make engine startup failure non-fatal in WinBoard

Make engine startup failure non-fatal in WinBoard

Unlike XBoard, which forks first, the parent process WinBoard has direct
knowledge of a failure to execute the engine command, and used it to
trigger a fatal error. The code doing this has now been disabled. Only
in case of a first engine we switch back to -ncp mode in stead, but no
action is taken to 'bury' the deceased engine process. Like in XBoard,
errors in writing or reading the pipes will trigger this.