Prioritizers enhancement

Prioritizers enhancement

1/ add the 'prio_args' config keyword to allow passing arguments
   to the getprio function

2/ merge the datacore prioritizer. Adapt the legacy datacore
   prioritizer callout to the libprio framework. First use of the
   'prio_args'

3/ fix the 'show config' multipathd cli command to display the
   prio and prio_args values. Also fix a bunch of other values
   affected by the same bug (features, ...).

4/ update docs

5/ remove some heading whitespaces

6/ remove useless prioritizers include files