scd: Rewrite READKEY to allow for compressed points.

* scd/app-help.c (app_help_pubkey_from_cert): New.  Taken from 2.3.
* scd/command.c (cmd_readkey): Rewrite using new helper.

Actually the readkey functions needs to return the uncompressed points
but if there is no readkey function, like in app-p15.c, readcert is
used and here we need to extract and the key and uncompress the point.

Noet that the --advanced flag did not and still does not work if the
key is fetched via readcert.

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