Parse zypp.conf multiversion option and make the setting available in pool and resolver.

Parse zypp.conf multiversion option and make the setting available in pool and resolver.

- Let  return the plain config file values (name or provides:...).
- Add 'bool multiversionInstall()' method in Solvable, ResObject and Selectable.
- Add sat::Pool::multiversionBegin to iterate over all multiversion installable packages.
- Pass the multiversion settings to the solver.