Sanitize pgpsigFormat()

Sanitize pgpsigFormat()

- Eliminate bogus size calculations: we have a buffer of td->count size
  that may or may not contain legal OpenPGP signature. Leave it up to
  pgpPrtPkts() to validate & figure it out and check its return code instead,
  eliminating need to repeat a bunch of tedious calculations here.
- Use non-zero signature version is used as a hint for valid signature,
  should be "close enough" for the rest of the code.