Attempt to fix getting trapped in ponder mode

Attempt to fix getting trapped in ponder mode

HaChu was sometimes trapped in its ponder search, because the pending
input that should wake it up out of ponder (i.e. the opponent's move)
had been read from the pipe to the input buffer, hiding it from the
test for pending input. Buffering linewise might prevent commands
to be buffered before HaChu actually starts to read them.