pkgman: Add full-sync command

* BSolver/LibsolvSolver: Add FullSync() method. It uses libsolv's
SOLVER_DISTUPGRADE mode.
* BPackageManager: Add FullSync() using the new solver mode.
* pkgman: Add full-sync command.

The new command is similar to the update command without arguments, just
more aggressive, allowing downgrading or even removal of packages, to
match the state of the repositories. Just like "update" it doesn't work
properly yet.