Remove support for V3 public keys

Remove support for V3 public keys

- V3 keys have been long since deprecated and extinct for all practical
  purposes for more than a decade by now (for example, Red Hat Linux 6.0
  from 1999 was the last RHL to use a V3 key for signing). RFC-4880
  says V3 keys MUST NOT be generated (they have a number of weaknesses),
  but implementations MAY accept them. We choose not to accept them
  anymore, eliminating a code path that would essentially only get
  triggered by malformed packages. The said code path also contained
  a few buffer overflows and other bugs, so its more than just
  "good riddance."
- Worth nothing is that only support for V3 *keys* is removed, V3
  signatures are still supported along with V4 ones.