scd:p15: Make RSA with SHA512 work with CardOS.

* scd/app-p15.c (do_sign): Rewrite.

This basically works now but for my test card I need to pass
--cert-digest-algo-512 manually. Need to check this again and
automate it. It will also be useful to take the NotBefore timestamp
from the dummy X.509 certificate and use that for the PGP key creation
time.

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