When multiple lei(1) processes are starting in parallel without
lei-daemon already running, it's possible for them to trample
each others' socket path trying to start lei-daemon. Lock
errors.log before unlink/bind/listen. We'll add an extra
connect(2) attempt to check if the starter lost the race.
Without this change, a stress script like the following could
easily cause problems:
lei q -o ~/tmp/a foo ... &
lei q -o ~/tmp/b bar ... &
lei q -o ~/tmp/c quux ... &
lei q -o ~/tmp/d baz ... &