plug-ins: create file-pdf-save2 with the additional parameter.

And revert file-pdf-save to its previous signature before commit
d03de527. For compatibility reason, we can't change the signature of a
procedure available in previous stable releases.
Also revert file-pdf-save-multi for the same reason. I don't create a
file-pdf-save-multi2 though even though layers-as-pages could still be
meaningful there (creating a multi-page PDF from several multi-layers
images) but that's likely a special case which can be dealt with later.
Obviously the interactive export will use file-pdf-save2.