gpg,gpgsm: Hide password in debug output also for asked passwords.

* g10/call-agent.c (agent_get_passphrase): Call
assuan_begin_confidential and assuan_end_confidential.
* sm/call-agent.c (gpgsm_agent_ask_passphrase): Ditto.

The drawback of this solution is that we don't see any IPC lines from
the assuan_transact. Everything else would require larger changes to
libassuan.

• GnuPG-bug-id: T6654