core: Fully implement the inquire callback for assuan_transact

* src/engine-assuan.c (inquire_cb): Implement returning data.
* tests/opassuan/t-command.c (inq_cb): Send some test data.

The old code only allowed to send an empty response which was good
enough for scdaemon's KNOWNCARDP inquire but not to send actual data.
A quick test using a test smartcard might be

./t-command 'scd setattr --inquire PRIVATE-DO-1'

and then reading the data back using

gpg-connect-agent 'scd getattr PRIVATE-DO-1' /bye

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