pdfsig: Sign form signature fields with an appearance
The last patch in this set adds support for pdfsig to add the "appearance" to a PDF document when signing an existing signature field, for example:
The patches leading up to that fix various issues I encountered trying to do this:
- using a signature field that's not attached to a form (patches 1-3) [doing this to work around Issue #1280]
- obtaining the signer name when signing_cert is set and CMSSignerInfo is not (patch 4)