scd,openpgp: Switch key attributes between RSA and ECC in writekey.

* common/sexputil.c (get_rsa_pk_from_canon_sexp): Also allow private
keys.
(pubkey_algo_string): Ditto.
* scd/app-openpgp.c (do_writekey): Switch key attributes

The scd WRITEKEY command for OpenPGP cards missed proper support to
aautomagically switch key attributes based on the new key. We had
this only in GENKEY.

• GnuPG-bug-id: T6378