[libprio] initial commit

[libprio] initial commit

Priority callouts are nice, but have drawbacks.

1) multipathd holds a fd per path, but callouts have to open another for themselves. Which may fail.

2) callouts stored on a multipahed filesystem may hang multipathed on paging operation when there are no active path for the hosting device (getprio is in the salvation codepath). The window is small but Netapp fault injection tool manage to triggers the bug reliably.

This patch merges to priority methods in multipathd.