Filter out duplicate problems when adding to element problem sets

Filter out duplicate problems when adding to element problem sets
- Problems associated with a transaction element are necessarily unique
  to that element, so when filtered there we don't have to worry about
  skipping dupes elsewhere like in merged sets. This can actually lead
  to apparent duplicates in the current problem report output (eg in cases
  where multiple packages provide the same dependency which would be
  removed, like multilib packages), but this is only an artifact of
  they way the problems are currently printed out.
- While this is still a dumb linear search, it can be several seconds
  faster than the previous filtering in rpmpsPrint(), which is now
  just a dumb convenience function.