Record problems into the transaction elements causing them

Record problems into the transaction elements causing them
- Any problem found in transaction processing is caused, or at least
  triggered by, some transaction element. Having the problems in
  elements lets API users easily determine which package caused which
  problem instead of having to parse problem strings in vain.
- No problem set stored in transaction set itself now, rpmtsProblems() API p
  reserved by making it collect problems from our transaction elements
  into a new set. The problem set fiddling in rpmtsRun() wrt %pretrans
  and all is a bit crazy - always was, now its just more visible...
- Make rpmtsCheck() pass the associated transaction elements into
  the lower level functions. rpmdsProblem() removed from API, it's
  not particularly useful outside rpmtsCheck() internals.
- rpmts struct is now opaque within depends.c except for solve callback
- This breaks --badreloc, need to figure out something saner for the
  problem filtering