Fix the usage of fcntl(2)

     Change: bdb-bugfix-fcntl-usage
     Purpose: Fix the usage of fcntl(2)
     Reason: POSIX/SUSv3 clearly expresses that fcntl(2)
                     returns "value other than -1" on success, so check
                     for equality to -1 instead of non-equality to 0.
                     In practice this does not harm on most platforms,
                     because they fortunately (by conincidence and
                     common practice) return 0.