scd: New standard attributes $ENCRKEYID and $SIGNKEYID.

* g10/call-agent.c (agent_scd_keypairinfo): Use --keypairinfo.
* sm/call-agent.c (gpgsm_agent_scd_keypairinfo): Ditto.
* scd/app-openpgp.c (do_getattr): Add attributes "$ENCRKEYID" and
"$SIGNKEYID".
* scd/app-piv.c (do_getattr): Ditto.

We already have $AUTHKEYID to locate the keyref of the key to be used
with ssh. It will also be useful to have default keyref for
encryption and signing. For example, this will allow us to repalce
the use of "OPENPGP.2" by a app type specific keyref.

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