If using --log or -j > 1, disable stdin.

Parallelism and redo-log cause lots of confusion for any rules that try
to ask the user for questions, so disable it altogether.

Arguably, we should just disable stdin all the time, but maybe it's
still occasionally useful (even though you have to pass --no-log to get
it back).

builder.py

@@ -22,9 +22,14 @@ def _try_stat(filename):
 log_reader_pid = None
 
 
+def close_stdin():
+    f = open('/dev/null')
+    os.dup2(f.fileno(), 0)
+    f.close()
+
+
 def start_stdin_log_reader(status, details, pretty, color,
                            debug_locks, debug_pids):
-    if not vars.LOG: return
     global log_reader_pid
     r, w = os.pipe()    # main pipe to redo-log
     ar, aw = os.pipe()  # ack pipe from redo-log --ack-fd