Pass keyring, not the full ts to lowlevel signature routines

Pass keyring, not the full ts to lowlevel signature routines
- lazy loading of default keyring on ts iteration works nicely for
  everything but package signature verification which doesn't access the
  db now that the keys are not in the db anymore
- make rpmtsSetKeyring(NULL) load the default keyring, a bit ugly but
  it'll have to do for now