[libmultipath] don't re-select when not necessary in path discovery

[libmultipath] don't re-select when not necessary in path discovery

.getprio, .getuid and .checkfn were needlessly re-selected each time the
corresponding value was updated through discovery.c:pathinfo().

Now that we are more selective, we need to :
- flush the cached values for multipath(8)
- re-select them in case of multipathd(8) "reconfigure" CLI command