Fix handling of session user module custom handlers.

According to the documentation, returning TRUE from
user based session handlers should indicate success,
while returning FALSE should indicate failure.

The existing logic relied on casting the return value
to an integer and returning that from the function.
However, the internal handlers use SUCCESS/FAILURE
where SUCCESS == 0, and FAILURE == -1, so the following
behavior map occurs:

  return false; => return 0; => return SUCCESS
  return true; => return 1; => return <undefined>

Since the session API checks against FAILURE,
both boolean responses wind up appearing like "not FAILURE".

This diff reasserts boolean responses to behave as
documented and introduces some special handling
for integer responses of 0 and -1 so that code can be
written for older and newer versions of PHP.