use flock instead of fcntl locking

use flock instead of fcntl locking

We do not need range locking of fcntl locks, so using flock removes
a dependency, hopefully making us easier-to-install.  Also keep in
mind Ruby (and perhaps other scripting language) supports flock
out-of-the-box as well, so it seems flock is easier to support
although fcntl locks offer superior functionality.