gpg: Fix export of NIST ECC keys.

* common/openpgp-oid.c (struct oidtable): New.
(openpgp_curve_to_oid): Rewrite and allow OID as input.
(openpgp_oid_to_curve): Make use of the new table.

Due to the previous change we now usually store the OID with the
private key and not the name. Thus during import we do not anymore
need to map the name to an oid but can use the oid directly. We fix
that by extending openpgp_curve_to_oid to allow an oidstr as input.