Create .gitignore file
Implement automated error code extraction

* src/gpg/.gitignore: ignore files generated by the error-code-snarf
  script

* src/gpg/error-codes.scm:
* src/gpg/error-sources.scm: delete as the contents of this file are now
  automatically generated and thus should not be versioned

* tools/err-setup.scm: delete due to obsolescence

* tools/error-code-snarf.scm: create error code generation script

* src/gpg/error-codes.scm.in:
* src/gpg/error-sources.scm.in: create input templates for code
  generation via error-code-snarf.scm
Complete error handling

* tools/error-code-snarf.scm: clean up output functions; output
  value->error-(code|source) lookup tables; bump copyright

* src/gpg/error-codes.scm.in:
* src/gpg/error-sources.scm.in: use immutable record types; make
  value->error-(code|source) lookup tables auto-generated; bump
  copyright

* src/gpg/errors.scm: complete error handling functionality