[checkers]

[checkers]

checker_lookup() silently returns the default checker if the requested
one can't be loaded. Even worse, it the default checker can't be loaded,
it gets stuck in an infinite loop.

It seems to me that if the requested checker can't be loaded, we should
just throw an error. That's what this patch does.

multipath.conf should be order independent, and with the existing
checker code, it isn't.  The multipath_dir had to be set before
you could set a checker, otherwise the checker load failed (since the
multipath_dir wasn't specified).