Introduce priorities for the solver.

Now all universe and request items has priorities. Priority is based
on the following principles.

- request priority is <repo_priority> (zero currently for all repo).
- dependency priority is <parent_priority> + 1
- reverse dependency priority is <parent_priority> - 1

Priorities are adjusted during universe processing, therefore
install/deinstall processes knows about the order of processing for
the solved job.
Merge branch 'master' of github.com:freebsd/pkg