Decouple dbiVerify() from dbiClose()

Decouple dbiVerify() from dbiClose()
- When RPMDB_FLAG_VERIFYONLY is set, dbiOpen() now only creates and
  returns the db handle without actually opening it. With this we
  can sanely implement dbiVerify() as its own separate functionality,
  also avoiding the need to have duplicate code for creating and
  tearing down the db environment (which in turn kills our ability
  to use DB_REGISTER if/when the day comes)