The problem shows up on 32 bit architectures where sizeof(time_t) !=
sizeof(gint64). Upcasting the 32 bit time_t to a 64 bit integer
should hopefully be safe.
The "key_id" field seems to used for userid in gmime-3.0, while the
keyid is dropped in the fingerprint field if the full fingerprint is
not available.
It turns out that our use of GMimeStreamPipe has only succeeded
because gmime has been ignoring some seek failures; this will no
longer be the case in gmime 3.0, so we use a GMimeStreamPipe, which
does not assume seekability, wrapped in a buffering stream.
