tdf#84881: Timestamp the right data (Win32 version)

Now Adobe Reader is satisfied with the signature timestamp also for a
PDF signed and timestamped on Windows.

My gleeful commit comment from yesterday about how much simpler the
Win32 crypto API was to use for this task was not entirely true. It is
simpler than using NSS and curl, but not as simple as I had hoped. Oh
well, I am not really surprised.

I now use the "low-level" message functions instead of the single
"simplified" CryptSignMessage(). And just like with NSS, I need to
create the message twice; first to get the signature to timestamp, and
then a second time to attach the timestamp. But now I wonder whether
doing CryptSignMessage() twice would work too. Anyway, won't touch the
code now for a while.

I am actually a bit surprised that the code works... Must have been my
lucky day. Or then I just am good at making educated guesses at what
an API does, even if the documentation doesn't make it perfectly
clear. Especially, I am a bit surprised that calling
CryptMsgGetParam(hMsg, CMSG_BARE_CONTENT_PARAM) returns (just) the
signature. OTOH, what else would it return?

Change-Id: Iec20c7605cf3d841b9e1787184c7b665837f1bc2