rename: DepSolveProgressCallback.pkgAdded()->DepSolveProgressCallback.pkg_added()
rename: base.dsCallback -> base.ds_callback
add default depsloving callback.

so base.ds_callback is always properly defined.
better streamline how conf.yumvar is created and initialized.

Namely, make sure it is always existing and initialized to the best of our
knowledge.

A nicer thing would be to get rid of it completely but that's not possible.
getter/setter for config.releasever (it is part of the yumvar).

yumvar should be less visible for 'casual API users'.