tdf#84881: Try to fix "The signature includes an embedded timestamp but ...

... it could not be verified"

I got some insight reading this question and reply on stackoverflow:
http://stackoverflow.com/questions/18761993/steps-to-include-timestamp-in-pdf-signature

I had been doing the timestamping wrong in the same way: I had timestamped the
hash of the PDF document, not of the signature. That is wrong. If you think
hard, it is obvious: It is the (rest of the) signature that needs an
authenticated timestamp, not the PDF document contents. After all, if the
document contents is timestamped, but not the signature, that doesn't prevent
tampering with the signature after the timestamping. When you timestamp the
signature, that proves the date of the signature. (And the signature proves
the authenticity of the document contents.)

So I had to re-engineer the code a bit. I create two originally identical NSS
CMS messages with signatures, encode one signature into DER, take the hash of
the signature, get a timestamp from the TSA for that hash. Then I add that
timestamp to the other CMS message as an unsigned attribute of its signature,
sign it, encode it, convert to hex, and store it the document.

(I first tried to use just one CMS message, but NSS stopped with an assertion
when I tried to encode the signature of the same message a second time, after
adding the timestamp attribute to the signature. Go figure.)

(I did verify the the encoded signatures, taken from what should be identical
but separate CMS messages, was in fact identical. So I am fairly sure the idea
described above is sound.)

But, it doesn't help. Adobe Reader still complains "The signature includes an
embedded timestamp but it could not be verified".

Change-Id: I4e4cd0443005e82f597586942badc7145ef64160