[Nmh-workers] locking of sequences file broken

[David Levine]

I noticed that locking of the sequences file was broken:  the
program would proceed anyway after failing to acquire the lock.
I have fixed that on the fix-locking branch; reviews would be
appreciated.

Ken observed that this might be the cause of the sequence file
clearing that you experienced with mark(1), Jerrad:

    http://lists.nongnu.org/archive/html/nmh-workers/2014-02/msg00155.html

The fix is to bail out, so hooks need to be prepared for that.

David