Add a brand new rpmkeys utility for keyring operations

Add a brand new rpmkeys utility for keyring operations
- Keyring operations (adding/viewing/removing keys and verifying
  packages against a given keyring) are different from main rpm operations
  in that they only need access to the rpm keyring, and no write access
  anywhere else in the system. At the moment the rpm keyring happens
  to be the rpmdb but that's just an implementation detail that is
  likely to change sooner or later. Besides paving way to separating
  the rpm keyring from the rpmdb, splitting this to a small, separate
  utility allows limiting its required access from SELinux POV etc.
- For now, this only implements what's already in rpm: --import and
  --checksig, remaining operations like listing and manipulating
  keyring contents is left as an exercise for another day...