Fix package relocatability checking, broken since forever

Fix package relocatability checking, broken since forever
- at the time relocation problems are added, ts problem set is NULL
  and rpmtsCheck() wipes it out anyway so the problems from relocation
  are never seen anywhere
- The header isn't available at transaction prepare stage where most
  problems are checked for, so we can't validate the prefixes at that
  point. Add a per-element problem set to carry the problem data and
  merge it to ts problem set when checking for other problem types.