[libmultipath] clarify priority values

[libmultipath] clarify priority values

Define two values :
PRIO_UNDEF: prioritizer not exec yet, or returned an error
PRIO_DEFAULT: no prioritizer set, default value arbitrary set to 1

Changes :
1) 0 is no longer used as a possible priority value.
2) don't exec prioritizer in pathinfo() if path is down and priority is
defined

Edward Goggin, EMC
Dave Wysochanski, Red Hat
& Me