In practice, we're going to see this function invoked differently
depending on which gmime we build against. The compatibility layer
forces our code into the lowest-common-denominator -- unable to make
use of new features even when built against a newer version.
Dropping the compatibility layer paves the way for clearer use of
features from GMime 3.0 in future commits.
"typedef GMimeAddressType GMimeRecipientType" is already present
further down in the compatibility wrapper (with other typedefs). We
don't need it twice.
When compiling as C code (instead of C++) against gmime 3.0, gcc gives
errors like the following:
error: invalid conversion from ‘int’ to ‘GMimeDecryptFlags’ [-fpermissive]
so use explicit *_NONE values instead.
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.
