keyboxd: Use D-lines instead of a separate thread.

* kbx/kbx-client-util.c (kbx_client_data_new): Add arg 'dlines'.
* g10/call-keyboxd.c (open_context): Set DLINES to true.
* sm/keydb.c (open_context): Ditto.

This allows to compile time switch between the D-line and the
fd-passing data communication between gpg/gpgsm and keyboxd. A quick
test with about 3000 OpenPGP keys showed that D-lines are only 10%
slower than the fd-passing based implementation. Given that the
thread adds extra complexity we go for now with the D-line approach.

• Signed-off-by: Werner Koch <wk@gnupg.org>