gpg,sm: Set confidential in assuan communication for password.

* g10/call-agent.c (default_inq_cb): Call assuan_begin_confidential
and assuan_end_confidential.
* sm/call-agent.c (default_inq_cb): Likewise.

Cherry pick from master commit of:
ec1446f9446506b5fbdf90cdeb9cbe1f410a657e

• GnuPG-bug-id: T6654
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>