Clean up pgpPrtPkts() and friends a bit

Clean up pgpPrtPkts() and friends a bit

- Use decodePkt() for added initial sanity check + grabbing the "main" tag
  instead of duplicating the tag decoding here.
- Call decodePkt() from the main parse loop instead of pgpPrtPkt(),
  and return simple ok/error codes from pgpPrtPkt() and do the
  length calculation in the main loop.
- Besides making the code simpler and more obvious this fixes some
  fishy cases where we previously would've returned 0 for success
  despite there being an error.