Created a new feature for the internal do_list() function if it
is necessary to immediately perform a function for each entry in a
list while the list is being traversed.  A callback function, and an
option callback data pointer, can be registered in the list_data
structure.  The address of each entry in the list along with the
optional callback data pointer will be passed to the callback
function.  If desired, the callback function may also dictate that
do_list() should stop the list traversal and return immediately to
its caller.
(anderson@redhat.com)