Reinstate MC6840 from old library (and add some rework)

Fix for #3119 (closed)

• Reimport the Motorola MC6840 from https://github.com/KiCad/kicad-library/pull/895 (in one commit)
• As follow up:
  • Reduce symbol size
  • Adjust pin offsets per newer KLC rules
  • Update derived symbol fields to say "PTM", not "PIA" (MC6821 is a PIA)
  • Update FP and filters in the new symbol (was DIP-40, now DIP-28)