Clarify the example by explicitly importing the fcntl module -- this

Clarify the example by explicitly importing the fcntl module -- this
avoid being fooled into thinking that fcntl and FCNTL are the same
thing -- they aren't!  (fcntl is the extension, FCNTL.py is h2py
output that defines all the constants).

(XXX The example is still weird -- I think there's a more portable way
to do locking now.  That's for someone else to fix...)