Handle erased packages wrt disk-space problem reporting (RhBug:561160)

Handle erased packages wrt disk-space problem reporting (RhBug:561160)
- erased packages in the transaction cause space needs to shrink, which
  commit 2222e81e1cf74bbf78acb9969592ab06e2289929 failed to take into
  account, causing false disk space errors from unaffected packages
- adjust obneeded and oineeded when the requirements shrink
- new disk space problems can only occur when the requirements grow,
  change the pmtsCheckDSIProblems() check to reflect that (although not
  strictly necessary)