sm: New option --issuer-der for the listkey commands.

* sm/server.c (do_listkeys): Implement new option.

This option can be used by clients who can only provide a DER encoded
form of the issuer. For example in PKCS#11 providers.

Testing:

Put the DER encoded issuer DN into a file, say issuer.der.
The run

gpg-connect-agent -E -- gpgsm --server
> /definqfile ISSUER_DER issuer.der
> list-keys --issuer-der 01020304

and if the local keyring has a certifictate with that issuer and a s/n
of 0x01020304 that certificate will be listed.

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