Add another pgpVerify variant which takes key and sig as separate args

Add another pgpVerify variant which takes key and sig as separate args

- pgpVerifySig() is now just a dumb wrapper around pgpVerifySignature()
  which does the real work.
- Update the sole caller to use the new interface instead, deprecate
  the old dig interface.
- First steps towards getting rig of pgpDig which always was a
  strange creature and now is nothing but a nuisance and obfuscation.
  Yes keys and signatures walk hand in hand much of the time, but
  they come from different sources and want to be handled as
  separate data really.