g10: Fix a regression for generating card key with backup.
* g10/main.h (receive_seckey_from_agent): Declare. * g10/keygen.c (card_write_key_to_backup_file): New. (card_store_key_with_backup): New. (do_generate_keypair): Create a key on host for encryption key when backup is requested. Then, call card_store_key_with_backup.
- GnuPG-bug-id: T2169